I just attached mine with loosely-hammered-in poultry staples. They lift up for moving real easy that way. I just did 2" x 4" welded wire for the aprons; predators reaching through the apron isn't a big deal (I mean what are they reaching for? Dirt?), so I didn't see a reason to pay for hardware cloth.
I just fold them up flat against the sides and I've got a couple of little clips on each apron that I just clip through the wire on the sides of the tractor. I'm not sure what they're called -- carebiners(sp?) or string or would work, too. Right now I don't hold them down with anything when they're down, though it would be wise to do so. Some folks use wire pegs to attach them to the ground. The only thing I wouldn't do is use something solid, like a board, since a predator might be clued in to dig *around* the apron instead of trying, and failing, to dig through it.