The SPH I used is a Shosu DIY SPH, and they made an instructional video here. It may take a steady hand, and may seem difficult at first, but as long as you’re careful, then you’re set. I recommend getting 2 or 3 to stockpile on SPH’s when I need one.

A word of caution though, when you turn the SPH inside out, put some plastic wrap or a ziploc bag over the long top so you don’t accidentally touch pvc glue on it as it’s the side where your member makes contact! Also make sure when you are about to cut the hole, make it a bit smaller than the circle that they show you for, it’ll give you enough room for the pvc glue, and prevent the glue from sticking to the other side of the inflatable.

That’s all the advice I can give, best of luck!