I look at pictures, work out a pattern in cardboard (I use cereal boxes, and renew the pattern once it wears), and keep patterns that have worked well. For a new item, the pattern tends to develop as I'm making the item, so I usually cut the leather generously so that I can shave it down, and the pattern will probably develop as I make the second and third item, until I get something that I'm definitely happy with.
Be aware, though, that leather and card don't bend in the same way, so if a card template folds snugly round, say, a knife, a piece of leather the same size will probaby be a bit short. I'd guess that the card represents roughly the middle of a piece of thick leather, with the inner face of the leather compressing a bit, and the outer stretching. I first discovered this when making a leather shot glass. I wrapped a piece of card around a glass, to get the pattern, cut a piece of leather the same size, but when this was wrapped around the glass, it didn't meet up.
I also tend to make pencil notes on the pattern itself, e.g. punch sizes that a I've use for holes, cost of materials, etc.
Geoff