Determine how your piece of driftwood lays best and decide where you want to drill holes. Using your forstner bit, drill into the driftwood being careful to not drill all the way through. Halfway is a good rule of thumb.
Step 2: Add Foam
Cut (soaked) floral foam to fit into hole. Be sure foam sits above hole about a quarter of an inch. This will give you more area to store stems and create a natural looking arrangement.
Step 3: Build Arrangement
Add stems and foliage and create arrangement. Take advantage of things you have growing in your own backyard. Foam will need to be watered daily for the life of the arrangement.