Things should escalate, like you go from touching the tits, down the body, squeezing tits a bit harder, teasing around the thighs, rubbing her ***** thru her jeans, taking the clothes off...Fingering...
It gets more heated as you go along, and since she's up for it as well, she'll start escalating as well probably like rubbing your d1ck through your jeans etc...and then you'll just do things more instinctively cause you're both getting horny and wanna ****.
Basically foreplay is just the stuff leading up to sex, usually starting with kissing, and it just escalates as arousal increases until you're having sex.