I posted a "plating 101" on the repairs forum. But I will give the cliff notes here...
Step one, hand polish the metal. The final finish well never be smoother and cleaner after the plating, than it was before the plating.
Put on latex or Nitrile gloves.
Clean the area you are about to plate to get all of the human oils off it. Whether it is hard rubber, celluloid, etc... will determine your allowed cleaning agent. Be sure to rinse with distilled water.
Mask and protect the non-metal parts of your pen barrel and cap. Some plating solutions can damage pen bodies and caps.
Based upon what you are plating, you may choose to do Brush plating, over dip or bath plating.
If Brass is the base metal, plate with Nickle first, then plate with gold... follow the instructions on your electroplating kit.
Polish the plated surface with a jewelers cloth... sometimes the plating looks undesirable when it first leaves the bath, a bit of polish reveals the beauty of the gold plating.
Remove the masking from the pen body and cap.
enjoy, admire... get a friend to pat you on the back.