I've tried adding different mods and cant seem to figure out how to activate them once in the game...or maybe i'm not doing it correctly.....I need help

Do you need to have a steam account for this? All the online tutorials i'm coming across are based off a steam account

is there a video tutorial because i think i might have messed this up, also how do i know if it worked?

I'm new to this and wondering how i download and install a mods menu? Also how i open it once in the game. Any help greatly appreciated!