I just received my new unpainted fender from Toyota with about 4-5 sq-ft of surface area.  I am planning on painting, then mounting on vehicle.  If needed I will blend paint with adjacent panels.

I ordered 2 spray cans of base coat, 2 spray cans of clearcoat, cleaner degreaser and rubbing compount.

Questions.

What prep work do I need to do on the panel? 
Best way to avoid drips using spray can?
Can the inside of the panel be left unpainted, not sure I ordered enough paint?
Is it best to paint panel vertically or flat?

Thank You

Please see:

https://www.paintscratch.com/touch-up-paint-directions/

Please practice  off the vehicle first. Use smooth even strokes to prevent drips.
Drips are less of a factor if you paint it flat.

Any unpainted surface should be primed. The inner panel should at least be primed.
You can order just primer here:

https://www.paintscratch.com/cgi-bin/storefront.cgi

Thanks for the info.

I forgot to mention that the fender is primed with a grey colored primer.  I am assuming I need to scuff with fine sandpaper (What would be a good grade for sandpaper?

400 grit is good.