To see how this works, look at Orion . You'll find that Orion is spread roughly between 4 hours 45 minutes and 6 hours 15 minutes of right ascension. Note also that right ascension increases right to left. So Rigel is about ten degrees west of Betegeuse. Now, looking at the left column of the graphics you find the Declension, in degrees.
Declension is defined as the angular distance north or south of the celestial equator.
As the declinations define a semi-circle from the south pole to the north pole, they cover a total of 180 . The celestial equator runs just north of the Belt of Orion, just south of Pisces and just north of Libra, right around the celestial globe. This is a difference of a little over 15.5 degrees. So you could describe Rigel as 10 west of Betelgeuse and 15.5 south. Circumpolar constellations take a little more thought.