How elaborative Test Strategies need to be is very context specific. Also: a Test Strategy is not a document but a mental model, an ongoing proces to guide your testing as I explained in the AMA session. Documents and visuals help you understand and transfer your ideas. A Test Strategy needs give insight in WHAT you gonna test and HOW. I made an example in my blogpost “Creating a Test Strategy”. I would not recommend using a template, because every situation is different. Also I am a bit reluctant to use templates, since they often become an exercise in filling in the blanks, instead of through (critical and creative) thinking about what we need.
If you need guidance on how to create a Test Strategy do the activities (read: build the models) described in my blog post. Be aware that the activities do not need to be done specifically in this order. Most likely you will do this in an iterative way, building your evolutionary Test Strategy as you go.
- Missions for your testing
- Product analysis
- Oracles & information sources
- Quality characteristics
- Context: project environment
- Test strategies
All these things can be found in the Heuristic Test Strategy Model (HTSM) by James Bach.
Also have a look at the interesting and well worded reply by Michael Bolton on this forum.
In the past I made my strategies visual in many different ways. Be creative and be concise. Do not write (heavy) documents if you do not have to. Only create artefacts that serve a purpose and add value to what you do. Most of the time people will not read them anyway. Some ideas to visualise your thinking: have a look at my collection of Great Resources on visualisation.