Sorry if this is explained poorly or if its confusing I'm bad at explaining things.
I'd recommend making a new game to test and play with mods, as they can possibly corrupt save files in existing games. Load up the game to see if it worked, to check if it did click the mods button on the start menu, if the mod you downloaded is there it worked and you should be able to use it. Go to files and manually download it, extract the file to the mods folder inside the SRML folder. If it worked close the game and go back onto nexus mods and find the mod/s you want to use and click on it.
Find SRML, it should be one of the first mods on the page, click on it and click files then click manual download (you'll need to make an account to download it dw its free), extract the zip file into your game file then load up the game to check if it worked, if theres a button on the start menu that says 'mods' then it worked.
Its quite simple actually (for me it was atleast), what I did was went on nexus mods (just search slime rancher mods it should be one of the first links if not the first link).
