Hello Crafty Tanuki...If you are securing the tree with brass screws, rafia is probably not necessary. You will need to treat the wood. First, if you want to bleach the wood more white, you can paint it with lime sulfur. Then you will need to soak the driftwood in some form of wood hardener/sealant. You could also just apply several layers of wood hardener instead of soaking it. As for the labor part... Basically, you would carve a channel in the back/side of the driftwood. Then strip away some bark from the live young tree where it is going to be attached to the driftwood. Then screw the tree into the drfitwood in a couple of places. You will also need to attach the tree to the bottom of the pot somehow. One method is just to drill a couple of screws in the sides of the driftwood. You would need to drill almost at the bottom so you don't see the screws. Then attach a mesh bottom to the pot, then wrap the wire around the screws and through the mesh and through the pot holes at the bottom to secure it. Some people actual use a kind of cement and create a base and then tie that in.
If I may...I do not know your experience level. However, if you are not experienced in bonsai or have not been in it for very long. This can be a very demanding project. Simply because of all the labor that is involved and then the tree will need to be craftfully wired into a bonsai shape. Usually tanuki are created by artists that have been in bonsai for many years..say 10 + years. You might find it better in the long run to gain a couple/few years of experience then attempt it. As I said, I do not know your experience level. Also, I would never try to discourage someone from attemptinmg a bosnai project that they are passionate about. I just want your final result to be one you are pleased with.
I hope this was helpful
Rob