It is worth having a read through the articles on this website before you start using your own money. Make sure you fully understand the concepts for BACK and LAY betting, as they are the complete opposite of each other.
Once you understand especially the LAY part, then start with small amounts and double check your figures before placing. Once you do this a few times, it will eventually become second nature to you.
You will make mistakes, in fact, people who have been doing matched betting still make errors every now and again if they are not concentrating.
Have a further read: