Best way is to make up a practise board as quite a few others will have done on this site. Go to screwfix buy a selection of different patress boxes and some 1.5,2.5 and some strapper cable, one way,two way, intermediate switches, lighting battons and some cheap sockets and just do it time and time again until you have it sussed out the way to wire lighting etc up. Also create the lighting faults normally found, perhaps even use junction boxes to wire in additional lighting off a main light (wall lights that sort of thing).
Use a cheap multi meter to check out the faults.
You can go a bit further and entirely at your own risk, put a 3 amp fused plug top on the this little rig to watch the lighting faults in action.
**Don't let anyone distract you when you do this and make sure you plug into a RCD protected circuit.
If someone gives you a bit of start latter you will have some of the very basics covered. The call from your mate who says he put up a light and it doesn't work because he wired it wrong.
