I think the hardest part is designing the sheath.
I've done the folding method with very little succes, and done the sandwich method successfully several times.
For me, the belt loop always comes out seeming like an after thought. If you make it integral to the sheath there is a good chance it will break off unless you it have just right. And if you rivet or use chicago screws to attach it, there's only so many place you can put them through.
So really think about what you want before you ever cut the kydex or start the heating !
The snap retaining loop on rlinger's sheath is cool, but you don't have to put one on yours. If you make the sheath cover a little more of the handle and press it right, you can make it as tight or loose as you want (I have a hard time finding the middle ground, where you can remove the knife without aid of a vice and it still doesn't rattle in the sheath

) -Not trying to say anything bad about the sheath Roger, I like it, just thats an extra step you don't have to tackle if you don't want.
Oh yeah, I mold both sides. Usually press the front, the press the back (not quite as tight, as when I go for the final fit). Stick them together around the knife so they line up, and rivet them together. Then heat and press the whole mess together (take the knife out while heating, and press TIGHT this time) Then trim things up, tweak the fit a little bit etc.