You can use hay cubes or hay pellets. If you're used to feeding grass hay, look for timothy, bermuda, or either mixed with alfalfa. Get a scoop or bucket and figure out how much by weight it will hold. If you have okay grazing, then supplement with 3-5 lbs a day, split in to two or three feedings. If grazing goes completely, or it's just dry short grass, you'll want to feed as much as you would with hay. For a 1,000 lbs horse, that would be 15-30 lbs a day. If you use alfalfa mixed with timothy or bermuda, you'll feed on the lower side.
Cubes provide more fiber and generally take longer to eat. You can spread them in an area in your pasture so your horse has to hunt around for them. Spreading them around will help prevent choke too, as those big dry cubes can cause choking issues if your horse bolts his food down. If you're worried about choke, you can wet the cubes down and let them break up for about 20-30 minutes before you feed them.