How do you make a leather guitar strap shorter?
To get it as short as possible, thread the strap through the highest slot, and then back through a lower slot, and then continue as it is now. You can start with going back through the third slot, and if that isn't short enough, through lower slots. For the absolute shortest, use only the first and last slots.
Which end of guitar strap goes where?
The end that's sewn directly to the nylon is the "bottom". As a strap it doesn't matter, but it makes it easier to adjust, and usually that plastic in the middle of the strap hangs to your side instead of digging into your back.