The Heuristic Test Strategy Model is developed by James Bach. You can download it from [his website](https://www.satisfice.com/download/heuristic-test-strategy-model). "You can use it casually or rigorously", he says in the description. I apply my own judgement and experience as a tester to gauge what I need from it in my current context. I would classify my use of it as right in the middle of that spectrum, most of the time.