I will try to lay it out to you in broad terms, if there is something specific you need help with I can try to help. Every mod can have different problems, so you need to be more elaborate than "fuck up my game and nothing works"
First of all, have a fresh install of gta5 and a full backup (ideally) - so that if you break something, you can copy the original files back.
Secondly, use openIV with mods folder like this. Really helps with not breaking the game.
Once you have your car mod make sure to read the README file before doing anything else. Usually eveything is explained in the readme file of each mod. If there is something you don't understand in the instructions of the mod, the you can ask here.
The general workflow for replacing a car in gta5 is:
- with openIV navigate to a specific folder (specified in the mod's README installation instructions)
- drag and drop the modded files (that you downloaded) into that folder and replace the existing ones. REMEMBER to have a BACKUP (like I told you in the beginning). If you have a backup it's easy to 'unbreak' your game.
More advanced installations include manual editing of meta files, but even then there are detailed instructions in README files.
Good luck! Ask if you need specific help.