I've found an easy way to get a neat fold is to run a ballpoint pen along a straight edge where you want the fold to be, on something fairly soft, such as newspaper or cardboard. Then fold the raised edge over, and press flat, either with thumbnail, dowel rod (as Bod suggests), or gentle taps with a rubber mallet.
The windshield that came with the WhiteBox stove was a bit too soft for my liking; I prefer windshield/pan supports, so prefer the stiffer foils such as oven liner/disposable BBQ tray (prior to use...).
I've only ever used a folded edge for strength, never finding that a cut, or scored, folded & snapped Al foil edge sharp enough to cut.
I don't recommend trying to fold Ti foil without experimenting first; my experience says Ti fractures very easily (work hardens very easily). A Ti fold is likely to make it very stiff, too.