If the receiver has pre-outs they are usually labeled as such. Be sure not to confuse the pre-outs with the multi-channel analog inputs.
Say you have 5.1 preouts on the receiver and are using a 5 channel amp. You simply connect an audio cable from the L pre-out to the L input on the amp, the R pre-out to the R input on the amp, and so on. You can use any audio cable you like or you can buy a set of cables that are color-coded according to the THX spec for each channel; ie L=white, R=red, C=green, etc (just to make it easy to identify at a glance which is which).