I reupholstered a chair I picked up at the thrift store. Making a cushion cover is not too difficult. It is basically the same as making a boxy pillow. The only "tricky" part would be if you are not comfortable inserting zippers, but even that is just a youtube video away from being easier.
I'm sure the previous posters have sent you to some great tutes. I just wanted to add that, in addition to using upholstery fabric, use upholstery thread. Thinner thread will either cut thru the fabric or pop with regular use. Also, I wrapped a layer of batting and a thin silky lining fabric over the foam cushion. The batting softens up the edges, and the lining makes the cover slide on easily. Since it is not seen, I just used a couple of safety pins to anchor it.
FYI, Home Depot sells the 3" foam in 24"x6' sheets for cushions at a quarter of the price of the fabric store. You do have to order it online tho. It's nice, comfy stuff that holds it shape well...I've been sleeping on it for 2 years, and it still feels like new.