Prepare the Wood for Finishing
Finishing wood begins with preparation. All wood should be clean, dry, and free from any protrusions such as nail heads. Any fasteners such as nails or trim screws need to be properly set and filled with wood putty. Use caution with wood filler. Use only small amounts and be sure to test it for color consistency with the stain. If you can not match the colors, then you will be better off just using a small amount of the polyurethane mixed with sawdust from the same type wood. Apply it very sparingly with a putty knife, and let it dry.
If the wood has to be finished in place and it meets a painted surface, then it must be finished first. Approach it just a like a good painter would; tape everything around it right off the bat.
Sand the wood next. Always use a high quality tool such as a high amp orbital sander for large even surfaces, or an oscillating tool for irregular shapes. Begin with a coarse grit sandpaper and progress to a finer one. It is important to not to destroy the profile of any shape. Stay away from hand sanding and small tools like a Dremel, unless you are planning on displaying the piece in a museum.
Use stain from a professional supply store. These higher grade products are just so much easier to work with. This is a case where it only costs a little more to go first class and its worth it. Just know the darker the shade is, the harder it is to get right. Never expect stain to fix any preparations not properly made. It will just make things worse.
The stain can be applied with a stain brush or a rag. Apply it with the grain and never stop once you start. Whether the stain is brushed or rubbed on with a rag, it should be allowed to set for a constant amount of time. Then it should be wiped down with a lean rag to even everything out and remove excess. Don’t worry about clean up. Just throw everything away.
This step will provide the wood with the important protection it needs to last. Just like the stain, always purchase the polyurethane from a paint store. The polyurethane has to be brushed on with a very fine bristled brush. Be prepared to spend a good deal of money for a quality brush.
The main thing here is to choose whether it will be an oil based polyurethane or a water based one. The water based products available now are almost as effective and are much easier to clean up.
If more than one coat is used, then be sure to lightly sand in between coats. Do not forget breathing protection while sanding. Have fun finishing your wood!