How can i stop the condom from slipping off during sex?

Trial and error: Just like clothes, condoms are all made differently. You need to try different ones until you find the one you like. But, have her use other back up. Because if it is slipping off, you are putting her at risk of getting pregnant.

Proper sizing: There are several different brands and sizes within those brands. You need to find one that is snug while you are erect , apply before you are, and stop if you go soft and it loosens.