Collect your driftwood. You'll find it near lakes in your area or on at the seashore during low tide. If it still has moisture from its time in the water, set it out in yourback yard on a sunny day to let it dry.
Drill holes in the driftwood to accommodate your plants. This is where creativity can come into play, as you can find pots as small as a couple of inches in diameter or as large as 1 foot or more. After you've drilled holes that are 1 inch or more wide, come back with a hammer and chisel to gouge out larger cavities.
Set your pots down inside the holes you've made to make sure the holes will allow the pots to sit level with some stability. If you have some pots that are too large for the planter box, but you want to show them near it, you can lean some smaller pieces of driftwood against those pots to make the decor consistent.
Take the pots back out and use the brushes to cover your planter with polyurethane. This will waterproof your planter box and protect it against the elements. Give the polyurethane time to dry; check the can for specific instructions about drying time.