When you launch Minecraft and click the mods button you should now see the mod is installed.
Place the mod you have just downloaded (.jar file) into the Mods folder.
Open the folder Application Support and look for Minecraft.
On mac open finder, hold down ALT and click Go then Library in the top menu bar.
On windows open Run from the start menu, type %appdata% and click Run.
Locate the minecraft application folder.
Make sure you have already installed Minecraft Forge.
This mod not required on the client (Although clients without it will not be able to go prone). GoProne Mod 1.17.1/1.16.5 just adds a keybind (default “C”) which sets the player to the crawling pose, allowing them to sneak through 1-block high gaps, without needing to squash something on top of them.