As a first time interviewer, I’m interested in your approaches.
Hey, you might find this post I wrote a while back useful: Tester Interviews: Techniques and Tasks | Cassandra HL
It describes various ways to interview testers, with some advantages and disadvantages of each.
Just some advice from my experience as an interviewer and interviewee: 
Let me share my journey as a first-time interviewer and what I’ve learned along the way!
What surprised me? How much preparation time I initially spent before each interview! This led to my first improvement: creating a reusable question database for QA interviews. Now I spend just 15 minutes tailoring these questions to each candidate’s CV - way more efficient!
Biggest challenges?
My advice for new interviewers:
Common mistakes I learned from (and still occasionally catch myself making):
This thread already contains lots of good guidance on specifics related to testing roles.
I want to pull it back to fundamentals: get very clear on the purpose of the interview, and then work backwards to make a plan that will help you achieve that.
IMO the goal of the interview is to make a clear yes/no decision on whether the candidate is fit for the role. So start thinking about what qualities are important, and how you can assess whether the candidate has those qualities.
I look to exit the interview with my decision already made. And if I’m waffling between ‘yes’ and ‘no’ then the answer is automatically ‘no’.
I really like the guidance from the Manager Tools podcast.
Caveat: your organization may have different expectations for hiring interviews. You should get familiar with what your org expects of you.
How much some people lie on their resume. It’s insane how easy it is to detect it.
I’m also surprised by some people who have 10years of experience in QA and actually know nothing.
It’s easy to build up questions and ask questions about peoples stories. But my biggest challenge once was a guy with 10-15 years of experience and he could just not even answer the basic questions right, so it kind of quickly became a firing squad of questions from my side, it was awkward since I couldn’t ask further questions on my basic questions.
Be yourself, the other side is just a human also. Try to throw in a few jokes and testing games to make it more fun! And if it’s no match, don’t drag it out, just tell them and move on.
Don’t forget to take notes to report back to HR 
Gauging (technical/coding and testing) competency. Resume only gives so much insight. Where I worked, QA/testers aren’t all focused on (manual) testing nor automation alone, so you have to find the right balance in evaluation of their coding skills and their testing skills.
Ask or come up with some questions that are more open ended from testing perspective, I focus the questions around seeing how creative or thorough (in coverage) the candidate is. Like you can test N cases for a given scenario, but there’s really no bounded limit beyond a certain minimum of cases to cover. Or there are different ways to test/tackle something and you want to see how the candidate approaches it. Some of this spills over to the automation/technical side in designing tests and frameworks, as you want to see how the candidate designs and models code/tests. Ideally you want something more thoughtfully designed and flexible than spaghetti code and rigid tests/code that only cover specific scenarios without rewrite to cover additional cases.
What surprised you?
Just how bad the overall quality of the candidate pool is. Particularly for SDET roles. I would understand if there were many candidates that were either great programmers but with a poor testing mindset, or great testers with poor coding skills, but I’ve interviewed an awful lot of candidates who absolutely stunk at both.
How did you become better at interviewing?
I actually started out as an interviewer with interviewing intern candidates, not necessarily just for testing roles. We had a set script of questions, intended to objectively measure candidates equally, but the more interviews I did the more I realized some candidates did poorly because of our script rather than because of their actual ability. My biggest “aha!” moment was interviewing a candidate with an impressive resume who did quite poorly on pretty much every question I asked. Since at that point I had gotten pretty skeptical of our script, I flipped the script and told him something like “look, I see these various accomplishments on your resume and I can tell you’re smart, but your answers to my scripted questions aren’t very good. Is there a particular CS topic you feel like you’re stronger on that we should talk about?” Sure enough, he suggested an area that we didn’t cover in our script, but I knew enough about to come up with some questions off the cuff, and he did great. Unfortunately, not all stories have a happy ending–I advanced him to the next round but this was in 2020, and all our intern positions were cancelled shortly afterwards due to the pandemic.
But I’ve kept that lesson with me–having an initial script can be helpful, and I constantly look for ways to improve it, but I’m not afraid to go off script particularly if I’m trying to find a candidate’s strengths.
What were your biggest challenges when you first started interviewing testers?
Asking questions that didn’t look for creativity or critical thinking. Plenty of candidates can give you a stock answer they read in a book (or <cough> the ISTQB syllabus), but disappointingly few can think on their feet and go deeper when you push them on context. Which leads into…
What advice would you give to someone new to interviewing testers?
Focus on creativity and critical thinking. I ask several questions that are intentionally badly worded, because I want to see if the candidate will think deeper and challenge me, or just go along with answering the wrong question. For example, in one question I describe a “simple” program and then ask “how many test cases would you need to test this program?”
Good candidates will ignore that specific request for a number and just start brainstorming, quickly realizing that the “simple” program isn’t so simple. Great candidates will start challenging, asking questions, pointing out flaws in the program design/requirements (and I throw some softballs there–it’s amazing how few candidates point out that using a single text field to accept three distinct input values is a really bad idea).
What are some common mistakes people make when interviewing testers?
Well, if the “top 100 interview question” cheat sheets on sites like LinkedIn are any indication, a huge mistake would be focusing too much on arbitrary definitions rather than, as I keep saying, looking for creativity and critical thinking.
Another I would postulate is not digging deeper. I get a lot of candidates that might provide a reasonable initial response, but if I try going deeper or asking the question a different way to see if they think about another aspect, they basically just keep repeating what they said the first time. Even if I specifically ask something like “what else would you consider” or “is there any situation where that wouldn’t be a good idea?”
THIS!!! It is sooo shocking, Like I understand that our industry can allow many to fly under the radar and it gets many of us a bad rap but my lord the amount of people who just show up to do nothing is astonishing. Especially when SO many of us absolutely love this craft.
But my advice and mistakes I made in initial interviews : Critical Thinking and Communication.
You need to know how the person can communicate in a difficult situation or any situation honestly. I found that during lockdown some employees did not do very well with with offline communication and others thrived. Also if something happens during testing do they figure it out and communicate it or blow up that something did not go as planned. How they react to situations can affect the entire team. These things never really occurred to me when I initially began interviewing people in my earlier days.
What surprised me:
How scary it was. I didn’t feel I was “good enough” to be on the other side when I started. I got better at interviewing through Toastmasters as I learned the skill of thinking and talking on the spot through table topics.
Biggest challenges:
Going with the flow of the conversation vs making sure to ask questions I want to ask - and managing time around this.
Also, knowing when to dig deeper. Now I feel I’ve got a hang of it, but before, I was a lot more uncertain on this.
Advice:
Remember that interviewing is a two-way street. THis means you also need to impress them. Make sure you are on-time, show some enthusiasm, make sure they know that there is time for questions in the end etc.
Make them feel comfortable, you want them to show you their best selves (so you have a better chance of choosing the best candidate). Also note that interviewing (on both sides) is a skill in itself. Someone can be a great fit and interview poorly and vice versa IMO
I feel it’s important to ask for examples and to get an idea of past behaviour. (I think I read somewhere that past behaviour is a good indicator of potential future behaviour).
I also like to ask questions that try to find out two things, so there is the surface aspect then something I actually want to know. e.g. “What is the best decision you made at work in the past 6 months?” (the hidden aspect to this question is seeing what they value as “best”) another one is “when it comes to testing, what have you changed your mind about?” (As. people learn and grow I expect perspectives and opinions on at least some topics to change or evolve)