Just put the rom's on the root of your sd card, which is when you plug your phone into your computer, it will list it as drive (d,e,f etc.) The root of your sd card is whatever opens when you click that drive in your computer. Copy the .zip file of the rom there and you are good to go. I currently have 4 on my sd card. To switch between them just boot into recovery by holding down the home key as you boot your phone, wipe first just as the rom developer says, make sure you have a nandriod backup, then just load the rom from the .zip file. Easy.