For the person who asked:
A "prefold" is still a rectangle, it's just been sewn to have multiple layers, especially down the middle where the baby will (usually!) pee/poop.
Practice on a doll if you don't have a baby available.
You put the diaper down flat and unfolded. Put the doll/baby down on the diaper so the back edge of the diaper goes where you'd expect the back of pants/diapers to hit on the baby's back (above the bum!) Take the two front corners, currently under baby's knees/feet and fold them in to the middle of the front edge so you have a piece about 1/3 as wide in the front as the unfolded back is wide. On a tiny baby, you will probably also fold this thick front piece down a few inches or else the diaper will be up to baby's armpits in front.
Holding the thickly folded front in place, use your other hand to bring the back corners around over the front to form the "shape" of the diaper around baby, right where disposables' velcro would be--ideally, only cloth is touching your child and there are many layers that would actually make it hard to pin your baby if you use pins. Use pins (through the top layers only) or a snappi to attach back edges over front thick layer.
I've actually just gone back to prefolds on my 3 year old! He outgrew his large Fuzzi Bunz, and I won't buy any larger diapers as I really think he's on the verge of potty training. I found them trivially easy in the beginning, but prefolds became VERY HARD to use when DS got super wiggly at around 6 months.
I still say, until you meet your baby, it is best to buy a few each of DIFFERENT TYPES of diapers. I was surprised by which ones I liked best sometimes!
I tried using flats (seriously--just an enormous, thin piece of cloth), and never could fold them well enough to fit my son, but I tried them after he was wiggly. (Newborns don't move much.) Ditto "Australian nappies" (the terrycloth flats.) Still, a flat diaper folded in half, then into thirds, is now my favorite doubler since one prefold won't hold my son's super-floods. Flats dry SO fast. I'm glad I only bought three of them, though, since they didn't work as diapers for me.