The prep work is the most important step.... wear rubber or latex type gloves....
All the oil & grease must be cleaned off....
I was told the good parts stores have Auto prep cleaners that are easy to use for painting prep.
Disassemble gun and mask off parts/areas that does not need paint to get on....
Spray a base coat of his choice color.... Krylon, Magic, Parker, Rust-oleum (sp) are good paints to use...... there is also a coating from Brownells.com called Aluma-Hyde that works very well.
After spraying the base coat.... take a sponge (can be found at Lowes or other places) and spray some paint into a paper plate, dip the sponge into paint and dab it on the gun.... Cut sponges in different shapes to give you different patterns.... Use the dark colors sparingly and they will stand out more.... and don't apply too much pressure or the paint will run.... just dab.
It's kinda just add colors and design the pattern to suit you.... for a longer lasting finish, spray a coat of matte finish clear coat over the painted areas.... let stand for a week or two so the paint fully cures. I would of course paint in the proper temps and humidity so the paint cures best if possible.
Here is an 870 a guy on OldGobbler just finished, I think it turned out great.... going by the hue, I would say he started with a green base color: