Hey there, if you are looking for something simple and cheap hit up your local Tandy and pick up one of there economy skins of your desired thickness and a pack of double end cap rivets. YOu can make your patern out of bristol board. You can either do a back and a face as 2 separate pieces or you can do a foldover style in which case you may want to soak the leather in some warm water, fold it then press it flat.
For a belt loop you can either use an extra length on the back side of the sheath and fold it back or just rivet on a separate piece. Just make sure you do this before closing the sheath.
As for holding it together, you can buy a rivet setter, or just do like I do and use a vice. Drill your holes in the overlap around the sheath using a bit the same size as the rivet posts. Feed the post in, press the cap on, then squich the 2 together in a vice, place it on a hard flat surface like some scrap steel and whack it with a hammer, or use the rivet setter.
Good luck to you.
-Chris