Cover your work area with drop cloths to avoid paint and varnish drips from damaging any flooring or plants (if you are working outside).
Sand your item, if desired (this is not typically necessary in the case of fence boards and other outdoor applications), to remove any rough spots or burrs on the wood.
Apply primer to the wood with the paintbrush (only if your finished item will be painted).
Apply an even coat of your chosen wood treatment (paint, oil, polyurethane or others) to the wood with a paintbrush or other paint application product.
Allow the covering from Step 4 to dry overnight, and re-coat.
Top coat paint or stains with polyurethane if desired to improve durability.