It's hard to explain without showing how to tie a western latigo strap. So check out this site... it has some good diagrams. How To Tie A Western Cinch
They way most people do it is through steps 1-6.
As for skirts, the only difference is that roud skirts are smaller & cover less of the horse, which lightens up the bulk. They also fit a wider range of horses as they're better for shortbacked horses.
The square skirts are used most for showing as well as bush riding, as they add more coverage for the horse & give more space for packing gear. Most Abetta saddles are square, but they're not as stiff as leather.
Here are some diagrams. The 2nd is a square skirt & the 3rd is a round: