This part is really important, because you don't want your slide crumpling beneath you, causing quite possibly the worst grass burn of your life. (I tried it. Don't be like me.) I used six stakes here, and distributed them throughout the entire plastic cloth (three on one side, three on the other, evenly spaced). I then made sure they were securely lodged in the ground. You can use a hammer, your body weight, whatever you have on hand, just be careful! You'll want to try to get the stakes as far into the ground as possible in order to avoid getting hurt on the dull plastic that's exposed aboveground.
Image Source: POPSUGAR Photogray / Taylon Faltas