My brother just got a Piper Arrow with a Sandel 3308 HSI. I can’t find any videos on how to use the 3308, but there are lots on how to use an HSI. These are some of the best.
For those of you who like to read, rather than watch an explanation, this Introduction to HSIs is a good start.
This video explains how to use the setup to figure out how VORs and HSIs work. Lots of tricks to figure out where you are based on the TO/FROM indicator and the location of the needle. For some reason, I never ran across these tricks before, but they are quite handy.
If you don’t have a sim, you can check out the basics of an HSI on the website from Luiz Monteiro that was references in the above video. Nav Sim (Requires Flash). Be sure to check out the instructions so you can see which keys move the wings.
There are lots of HSI questions on the writtens, and most of them have no relation to real-world flying. Here’s a video that helps explain how to answer these questions.