How do I use the old book version of the Encyclopedia Britannica/Americana?

How do you use the old book (not online) version of the Encyclopedia Britannica? When I used to go into a library there were two sets on the shelf. One was called (I think) Macropedia and the other Micropedia (does that sound right or have I confused them?)....I just used to look up things in both without ever really knowing what I was doing. How should I have - how should I - look up things in a library with it?