The exact procedures may vary depending on what kind of paint you are going to use.

Clean oily residue using acetone as a solvent. The blue industrial paper towels made for shop use work well for this. Dampen one towel with solvent and hold a clean dry towel in the other hand. Work small areas at a time as acetone evaporates quickly. Wipe with the damp rag and follow with the dry towel.

Once degreased, sand any thing to be painted with about 200 grit paper. Blow off with compressed air and repeat wipe down with acetone procedure. Sanding isn't 100% necessary, but will give a better bonded finish.

You're degreasing before sanding, because sanding can grind oils down into the pores of the metal. Your sanding to give the surface a "bite" for adhesion.

The final cleanup removes any residue from previous steps. Use a primer recommended for the type of paint you are using.

Some people skip this or use a different color paint. Primer bonds to both the paint coats and the metal. Also primer is primer, paint is paint.

Some of the automotive and industrial finishes such as urethane are more durable than enamel, but can be quite hazardous to your health to spray unless proper protection is used.