You have to find some way to get it across the cartridge terminals.
Our preamps have a load strip on the back so its rather easy. Other preamps may only have settings available from the front panel. If that is the case, you'll have to get some connectors too so that the connection can be made prior to the input of the preamp.
The middle pin and one of the outside leads of the potentiometer are wired together, the same in both channels. The resulting connections are somehow made to connect one to the + output of the cartridge and the other to the - output such that the pot (now wired as a rheostat) is in parallel with the cartridge.
If any of this seems tricky to you, you're going to need some assistance when you set this up.