Cheating on your partner is a pretty serious decision to make, one that should be avoided if possible. But if you decide that you are going to do it, you should do it right to decrease the possibility of someone getting hurt. See Step 1 below to get started.
Use an online dating site, to help you find a partner from the privacy of your own home. There are even websites specific designed for people wishing to have extra-marital affairs. When browsing online, make sure to use the 'private' browsing feature in your web browser to make sure that your web history doesn't get recorded. You should also download an application to delete any cookies, history, etc. from your computer.
If your partner brings up suspicions they have, keep it simple, give them a lie they can hold onto, and then stop leaving evidence. Cover it up and don't make the same mistake again.
The more trusting and forgiving your partner is, the easier it is to lie to said partner, but try to keep the lying to a minimum. A mountain of lies no matter how well constructed is as good as proof in the end.
Don't get moody with your partner when you try to kick him or her out of the house to make room for someone else. Every suspicious act will bring you closer to getting caught or dumped.
It may be somewhat effective to occasionally act like you're suspicious of your partner's activity, as if he or she were the one doing the cheating. Be careful though, as this is recognized as a cheater's tactic.
If your partner makes a surprise visit and you barely have enough time to get the subject of your affair out of the house before your partner arrives, don't shrink away from his touch like you feel dirty or grumpy. If you act like you're happy to see him/her it's less suspicious than acting like you're pissed off that you were interrupted.
Cheating not only breaks hearts but it also break families, wipes away the happiness of peoples lives. And not only the life of the one who has been cheated but also of the one who cheated, in fact cheating often leads to depression, suicide and even killing sprees, so you could end up with innocent blood on your hands.
If you don't love him or her, then simply tell them the way you feel. Breaking up is the best way to get rid of the relationship that is bothering you, and the relationship that is eventually going to bother the other person as well.
If you do get caught your partner will be devastated. Surely the next step from your partner's side would be to go away from you by breaking the relationship, but if that really was supposed to be the end, then why shouldn't end it without anybody being felt ditched, guilty or liar etc?