1. Disengage the gears on the spool and hold the spool with your thumb. There is a lever or 'push button' to disengage the gears, depending on the model.
2. Make your cast as normal and release your thumb from the spool at the same point you would release the line if using a fixed-spool.
3. Stop the spool again just as the lead hits the water otherwise it will over-run as the lead slows down.
4. Release your thumb again so the line can run out until the lead hits bottom (otherwise it'll swing back in and waste some of the distance you presumably put some effort into achieving.
If you are new its best to have plenty of braking ... either the centrifugal or magnetic sort and/or minimize the end float in the spool. It also useful to hover your thumb over the spool in mid-cast. You'll hear a birds nest starting to form (its a crinkly sound) and if you have your thumb at the ready a slight dab will prevent a disaster.
It might be worthwhile taking another reel at first in case you do get a monster birds nest coz that can just about be the end of your days fishing otherwise, and of the line.
The good news though ... multipliers are much more easy and natural to use than fixed spools, once you get the hang of them.
The idea of taking another reel is a bloody good one also try and find a field and get a bit of practise in first as it is a bit more involved than a F/s cast and it's better to get the inevitable birdsnests out of the way when you're not wasting fishing time.