As for the birch bark first you have to harvest some, late spring or summer, cut around the tree with the circumferece, peel it off and seperate it
into layers you don't want the full thickness just a thin layer. Size the back of the bow with TB III, lay the bark on, then I clamped at the nock and worked the air out to the other end. Then I put a thin slat on top and clamped that down with light pressure, after about 30 minutes remove the clamps and trim to size with a razor knife.
As for sealing I used TruOil but don't think any sealer is necessary as Birch Bark is waterproof. This bow took 4 pieces of bark I just butt joined them and wrapped the seam with thread.