Use a vacuum cleaner to suck the air out.

The fastest way to deflate a pool float is with a vacuum cleaner—specifically, a wet/dry vac or a standard household vacuum with a hose. Stick the hose over the valve (most valves have a small opening that fits a vacuum nozzle snugly) and turn it on. That giant inflatable unicorn will be flat in about 30 seconds. No, it won’t damage the float if you keep the suction reasonable.

If you don’t have a vacuum, the next best thing is rolling. Lay the float flat, fold it lengthwise once or twice, then start at the far end and roll tightly toward the valve, pushing the air out as you go. Use your knees or body weight to keep pressure on the roll. It works, but it’s a workout.

For folding: once the air is out, fold the flat sheet into a rectangle the size of your storage bin. Don’t bother trying to get every last molecule of air out—the vacuum already got 99% of it.

Your float will store flat as a pancake.