After the guy has come he has to pull out before his penis gets soft, holding the rim of the condom so it won’t slip off.
Take the condom off when there’s some distance between you and the girl’s vagina. Wash your hands to prevent the sperm from impregnating her later on.
If you want to make sure nothing went wrong, test the condom afterwards. Fill it with water, dry it off and see if it leaks. If this is the case and you want to prevent pregnancy you have to use the morning-after pill or an emergency IUD.
Don’t flush the condom through the toilet, for it will block the system. Wrap it in paper and throw it away.