There are a number of ways to do multitrack mixes. U can use a 4track, minidisc players, a cd player, and/or a computer (programs like acid and soundforge).
You can record a live mix and leave noted gaps of instrumentals (32, 64, 96... beats long) then play it back thru the mixer and mix on top of the gaps w/ acapellas, scratches or whatever.
With programs like acid, u can just put down beats and throw whatever u want on top of it.
I would suggest starting to make a list of what you like or what sounds good to you ... beats w/ certain vocals or whatever, cause depending on how fast you re going through songs, ideas might not come up right away.
I tend to start alot of multitrack mixes which never get finished cause most of mixes take sooo long for me to make and sometimes i get bored of it. An hour of work might produce like 3minutes of a mix... it all depends on how prepared you are and how fast your mixes are/# of layers.
Hope this helps!
