A few tips:
Double, triple,
quadruple check everything. If you've had a bookie account at a bookmaker before, they won't give you any signup offers. If in doubt, the big ones have an online chat helpdesk you can ask. Don't be afraid to ask as they will sometimes offer you a little incentive too.
Odds change all the time, make sure they match your spreadsheet or it'll mess up your finances (again, quadruple checking everything).
Make sure you're backing / laying the same bet. Make sure you're not backing / backing the same bet.
Read the Ts&Cs. Qualifier bets usually have to meet certain criteria. Read carefully and adhere to them or you won't get a free bet.
Stick to footie at first. Also stick to the more major teams / leagues until you understand it better.
Start with a couple of £5 or £10 offers and go from there.
I also use Topcashback. I'm on £52 extra so far.
Download
THIS
Enter all the details you can. Once you enter the back bet odds, back bet stake, and whether the bet is Stake Returned or Stake Not Returned, and finally the lay odds, it'll tell you how much to lay on the exchange (pink box if you're using Betfair).
Click on "add to records" and voila, your bet is saved. Wait for the game to play out, claim your free bet, do the same, and profit.
Any more questions just ask, I'm bored today
It gets easier once you get the hang of it. It's great fun, I think that's the more important factor for me, profit is just a bonus.