This wouldn't be easy at all. But if you want to succeed, you have to do this. Nothing will make your spouse more suspicious of you than being not yourself. He/she might not realize right away what you are doing behind his/ her back, but it will definitely open the door towards you getting caught. Just try your best to keep calm, do not be paranoid. Once you get the hang of it, everything will go smoothly.
Be clear about your intentions
Don't mislead another person with your touch and sweet words. If you want to keep it casual, be casual. No terms of endearment, no caring and thoughtful gifts. And also make sure that your partner-in-crime has the same intentions as you are too. You wouldn't want to get caught in the middle, and you wouldn't want to get busted because your other person told on you to your spouse.
Less talk. Less Mistake
Don't try to be too chatty. If you think that it helps when you converse with your spouse too much to cover up, actually it doesn't. Talk the right amount as you normally talk with him/her.
Never shit in your own backyard
Try to keep your affairs far from your home, your workplace, or anywhere that people might recognize you. Remember, it is not really helpful that you flash your illicit relationship in public especially around people who knows that you are married.
Never get pregnant/ get your other partner pregnant
Because let's face it- you wouldn't want to bring a child to this mess. First of all, it will be unfair to the child. Second, you will not be able to hide it anymore. But more importantly, the first one.
Don't use your cards on your dates
Credit card statements are like a paper trail that will rat you out. So better have a cash-in handy so that there will be no record, accessible by your spouse, that will point towards your affairs and extramarital activities.