Some preamps have built in DI's. Usually that shows up as a 1/4" jack right on the front panel. If so, you can plug a guitar or bass directly into the preamp (or channel strip too), and use whatever controls are on the box as if it were a microphone that was plugged in.
If the preamp does NOT have a built in DI circuit, you will need to use a direct box between your guitar or bass and the preamp. The direct box converts the signal coming out of the instrument to a level that won't overload the mic pre. Then, as far as your preamp is concerned, it thinks it is dealing with a microphone.