First and foremost once done it can't be undone. Most likely it will ruin your marriage and cause major pain to your closest family members. You will lose longtime friends. You will feel guilty and most likely this will be the biggest mistake you have ever made in your life.
That said if you have read my virgin bride to slutwife history post here… https://flashingjungle.com/terry-webb-virgin-bride-to-slut-wife-history/
than you know my story. It was an affair on my part that lead to the extreme slut I am today. There was a LOT of guilt and pain. I often considered suicide. When I begged to return to my marriage my hubby was deeply hurt. He lost respect for me and because he already considered me a slut the only way I was allowed back was to agree to do ANYTHING he ever requested. He had to actually see me as a slut with his own eyes before he could be sexual with me again.
He set up the blindfolded gang bang. To this day I have no clue how many people were there or who any of them were. I know there were a few women there but lots of guys all having a turn with me. I felt like a real slut. It was like being raped but my being the willing victim.
Fast forward now I am as extreme in the lifetime as possible. I want to set the world record for the most sexual partners in a lifetime. I open my legs as often as possible for all willing partners male or female. I love being the slutwife whore I am today. Nothing is taboo and I love being treated like a piece of meat. Sometimes I wonder if it is because of such low esteem in myself going back to that choice to have an affair. Life will never be the same now.
Now on to your question. Most affairs start in the emotional world. You grow close to someone other than your spouse until it seems to naturally be come physical. You can of course try to meet a woman just for sex thru on line sites such as adult friendfinder or many others. You can look for them in bars or other public places however it seems most affairs start in the workplace.
Good Luck but more so please give this some serious thought before crossing that line.