Take dinput8.dll and ScriptHookV.dll (and the trainer, if you want it) and put them in your GTA 5 game directory, wherever GTA5.exe is located. It allows for all sorts of cheats in single-player, like changing your skin, changing the time of day, teleportation, invincibility, and more.

In the zip file, there's a folder called 'bin' with three files in it. Just be sure to use the download link inside the table on the bottom-right: there are advertisements on that page that also say they are downloads.

Here is the link Blade provides to download ScriptHook. Most of the current mods are scripts, and to get them working you'll need the latest version of Script Hook, created by Alexander Blade.