If melonloader was correctly installed, it would create a userlibs folder after you launch the game once, then you can put the things in the userlib folder, but you could also create it yourself, the correct name is "UserLibs"
again, if you already launched the game after installing melonloader correctly, it should exist, so you're doing something wrong, Sirene's explanation isn't complete and you'll have to read a bit between the lines, but it's because it should be straightforward.
1: Launch meloninstaller, pick the executable of the game, untick latest, select version 5.7, meloninstaller will automatically read that the exe is 64 bits so you don't have to change that.
2: create the Userlibs folder, put universelib in, create the Mods folder, but the other shit in. (All these folders are going where the game's executable is)
3: Launch the game, wait like 10 seconds. melon will autolaunch unity explorer. press F7 to unlaunch it. then do Sirene's video instructions.
If melonloader doesn't autolaunch, something went wrong. you should delete the game and reinstall it and redo those steps. (just in case something went wrong, that's not a necessary step, just a safety one)