The guide is here, Scam Report Format (Use it to make scam reports properly). The button to create a new thread can be found on the top right corner of each board,
If you want to raise a scam accusation, then you're already on the right sub-board.
You can upload image of your proofs (screenshot, etc.) from imgur, imgbb, prnt, and the likes. The forum requires a newbie to buy a copper member to be able to show image, or you'll at least need to be a jr. member. But that's not really necessary, most likely than not, a highet rank member will quote your post to show the images