One of the first and most important things to consider when wanting to apply a durable finish of polyurethane to any wood is the kind of shine you'd like. Too often, people don't realize how dull or how bright a finish can make the item look. If it turns out too dull, then the item will look used up instead of rustic. If the shine is too bright, it can feel as though you are looking in a mirror.
There are a lot of things to consider when considering a durable polyurethane finish:
Type of wood: some will look grainier with a bright shine.
Type of item to be finished: for example, you wouldn't want your curio cabinet to outshine the precious items it contains, but you may want a high shine on an antique table sitting in the hall.
The most common kind of finish is a satin. This gives you plenty of shine without the overkill.
Once you have chosen the shine, you will need to acquire a few items along with your polyurethane.
First, and most important, is the application you are going to use. If you plan to brush on the polyurethane, make sure you choose a brush with soft bristles. Beware: most polyurethane is lacquered base. This will require an oil brush, not a latex brush which would make the urethane look streaky. A black china bristle brush usually works best. If you are planning to apply the polyurethane with a roller then you will need the smoothest roller you can find. Mohair is the best.
You will also need to buy some sandpaper. The higher the grit of sandpaper, the smoother. Around 250 grit will suffice. You will also need some paint thinner or mineral spirits, and some clean soft rags.
If the wood has recently been stained, make sure the stain is thoroughly dry. Pour some of the polyurethane into a wide pot or other container that is easy to work with. You should thin the polyurethane just a little; use about 1/2 cup of thinner for 1/2 gallon of polyurethane. Stir together. This will help the polyurethane go on more smoothly, and it will look more even. Be careful not to add too much thinner, as that would decrease the durability of the polyurethane.
Regardless of whether you are using a brush or a roller, you will want to apply the polyurethane with quick and even strokes. Try to go the entire length of the wood. This will minimize the amount of streaks or lap marks. Once first coat is completely dry, sand with equal pressure in a circular motion. Wipe with a clean cloth and apply a second coat in the same manner as the first.