How do I record my mic using a Rane SL box?

You will need an unbalanced out (RCA) output from your mixer (master or effects send will be fine) and run into an SL box input.

Use the record function in Serato DJ as normal, you will see the signal coming through the Recording Input that you have connected the Mic to.

If your hardware supports it; Aux is a good channel to send the Mic to, as Left and Right channels are better suited for the software decks.