Your ratio for mixing the clear is whatever the manufacturer calls for. In your case 2 to 1 on the clear to hardener and 1 to .2 on the thinnner. The thinner can be adjusted to your liking for spraying, but be careful, you can make it run far easier as it gets more thinned.
The base coat should always be as smooth as you can manage. No flaws, blips, hairs, fisheye, etc, those WILL show in the clear when done. Paint on cars is 90% prep work. Get the base cost smooth and error free, doesn't need to be gloss, just smooth. The clear will do the rest. Then clear with 2K. Remember, this is NASTY stuff and you must wear protection, vent very well and use a respirator. DON'T even spray this without precautions!!!
Decals go over smooth surface and under the clear coat. If you put them over it they will look like they float and will lift off over time.
Make sure your work is clean and free of oils and other contaminents, those cause fisheye, etc. I use pipetts to move 2K from container to mixing cup, then a NEW pipette for the hardener and again a NEW seperate pipette for the thinner and retarder as well. Never reuse anything except your Airbrush. Pipettes are super cheap by the truckload on Amazon, get some. Mixing cups are also super cheap. I use cocktail straws for mixing the paints and clears and toss them.
With clears you can kinda fog them on and to work correctly they must go on WET, you can't allow it to have a fade that dries and them keep spraying, it just doesn't come out very nice. Smooth wet overall, but not too much to run. It takes practice.
Pressures can very, but with my setup I use 20 ish PSI due to my syphon feed brush I spray my clears with. You can use less with Gravity feed brush. The key is to keep the edges wet as you work so the clear goes down nice and smooth. When your clear has hardened up over a few days or a week, unless you use a dehydrator, you can then wet sand and polish it out to a mirror finish.
Really nice auto paint takes a great deal of work in prep, paint, sand, repair, sand, clear, sand, polish, admire. There are no shortcuts. You won't like your work if you try to just through color and clear on it and call it done. There is far more work to nice paint than that. I spend more time on the paint and body then I ever do on the assembly and finish work. Good luck.
BK
PS a good rule of thumb is to get some plastic spoons and use them to test on. Paint them with the same primer, paint and clear you are using on your kit. When you get them to look like you want and you didn't screw it up. Then move on to your kit. Better to waste a spoon than strip a kit and start over. Ask me how I know.