I don’t really have a guide anywhere, but there are a number of them on the web. One of the OSINT powerhouses is BellingCat. They make finished products. There’s liveuamap.com/en for a map on the RU/UA war. Oryx is a good estimate for equipment losses there (I bet you can tell where my interests lie) I’d recommend getting to know some of these finished products first. That, and reading about the history of where you are looking at. Learning about the politics of Coal? Read some books about it. Get some perspectives. Want to know about the Isreal/Palestine conflict? Get to know the last 100 years of conflict since the Ottoman Empire fell
There’s a real big difference in bits and bites investigating and actual finished products. A lot of the tools out there are for getting these bits and bites like osintframework.com. One can buy commercial satellite photos, but those are expensive. They’re usually already bought by people on Twitter anyway. Putting together products is the hard part though, and there are quite a few pitfalls that one can fall into between unreliable sources and deceptive imagery persuasion or DIP. Ryan McBeth is a great source to look at to help you spot this sort of thing.