Park both cars in neutral next to each other so that they're close but not touching. Open the bonnet; the jump lead should be able to reach both vehicles. If need be, turn the working car round so the batteries are closer.
Connect the first end of the RED positive lead to the positive terminal on the working battery. Do the same for the other end of the lead for the flat battery.
Connect the BLACK negative lead to the negative terminal on the working battery. Attach the other end to the negative terminal onthe dead battery.
Check the leads are away from moving parts then start the engine of the working car.
After about a minute, try starting the car with the dead battery. If it doesn't work, leave it a little longer, but switch off the engine if the leads get hot.
Once you have your car running, leave the engine on for a four minutes to put some charge in the battery (if you remove the leads immediately the car has started, you risk damaging the batteries).
Remove the jump leads in reverse to the way you attached them. Make sure they don’t touch together or any metal surfaces. You should then drive for at least 30 minutes or leave the engine running for as long. I was always told it took at least 15 minutes to replace the charge needed to start the car in the first place. Longer if you are using windscreen wipers, heating fans and stereos.
If you only do lots of short journeys try to take the car on a longer run once a week, or at least run the engine for 30 minutes weekly.