You'll run a cable from the "EFFECTS SEND" on the 2488 to the Input of your effect. Then you take 2 cables and run them from the L and Right Outputs of your effect into 2 of the Inputs on the 2488 (Inputs E and F, for example). If it's a mono effect, then just run one cable from the effect to the 2488, obviously.
I don't have my machine any more, so hopefully someone else will help you with the routing after that. I think you would route the Input(s) into the Sub-mixer, but I can't remember for sure any more.