Can I play offline normally, and online after this Permanent Trainer Installation? I want to use cheats only in single player, and I don't want to uninstall my trainers.
The safest way to keep from accidentally playing online with mods is to have two separate copies of the game, one with mods, one without. Rename the folders when you want to swap.
Here's the guide most people refer to: https://steamcommunity.com/sharedfiles/filedetails/?id=558079253