Simply pour it over the item you have chosen to coat. You can color or tint the resin before you pour it, should you choose. Expose the resin to UV light for it to set. How long you need to expose it to the UV light depends on the thickness of the resin, and the size of the piece.
The application process varies, depending on the reason you are using the polyurethane resin. If you are making a mold casting with polyurethane resin you will need to combine the two components before you pour the resin into the mold.
One applies the resin layer by layer, allowing each layer to dry before applying the next layer. While you can sand down the resin once you have applied the initial coat, no more sanding should be done between any of the additional coats.
Polyester resin requires you to combine the two components prior to using the resin, you will need to add the hardener to the resin. The amount of hardener that is added determines the curing time of the resin. Once you have combined the two components you need to mix them completely.