A clear glass window and a clean fire chamber are sure signs that the stove is burning correctly. If your stove blackens, either the air intake is too small, the wood is too damp, or there is too little draw in the chimney (the chimney may be too short).
Make sure you use good dry wood. Logs should be cut to fit the fire chamber.
Use firelighter cubes. Start by lighting smaller pieces of wood, and then change to larger pieces.
The flue gasses in the wood are what generates heat. These burn most effectively at high temperatures. This is why it is important to keep an eye on the flames. Once they have a bluish tint and a slightly wavy shape, this means that the wood is burning at optimal temperature.