What to mix to paint a car detail

I’ve decided to replace one of my fenders of my car since it has rust. I bought a second hand one and i have to change its color.
I went to a paint store and the person there checked the paint mixed it and gave me a box full of items. I have a spray gun, and checked some articles how to do it myself.
The steps that I read are simple – use sandpaper 400 to remove the paint, use liquid to remove the oils, then add one or two layers of primer, wait 1 hour, use 800 wet sandpaper to even it out, remove oils again, spray the paint 1 or 2 layers, clean it and spray the varnish.

Now the issue is that I don’t really know what to mix with what. I have removed the paint and started mixing the primer but is was way too thick, I noticed there are 2 components – one is the primer , the other one is a small bottle that says hardener, so i mixed them together into the spray gun container, seemed to be working but is not spraying evenly, tried to adjust to spray gun nozzle not much luck, at certain point there were only bubbles into the container and air outside of the gun nozzle.
Now I think I will remove the primer with sandpaper 400 again and start from there.

I was using I believe thinner – corseline, to remove the oils is that correct way?
Also, how should I find out if i have to mix the primer with hardener and what proportions?
Should I thin the pain too, and how to thin it with corseline or water?
On the bottle of the varnish says 3:1 but it doesn’t say what should I mix it with, should I use corseline too?