In addition to reading the directions on the can, keep these basics in mind.
Test the old finish.
Acetone-based nail-polish remover softens lacquer. Water drops turn wax white in about 10 minutes. If either lacquer or wax are present, strip them before proceeding.
Stir the can.
Do this before and during application to evenly blend the ingredients that control sheen and UV resistance.
Apply thin coats. Thick ones take longer to dry and are more likely to drip or wrinkle.
Sand between coats.
Using 220-grit paper helps ensure good adhesion and smooths away imperfections.
Wipe after sanding.
Dust interferes with adhesion and leaves unsightly bumps in the finish. Use a cloth dampened with mineral spirits on oil-based finishes; use a water-dampened cloth on water-based finishes. A dry microfiber cloth also works well with both finishes.