Cut each individual flower and the little stem that connects it to the main stam....no need to cut them back right now.....and the need for cutting back will be delayed if you deadhead.
And while I am deadheading, I punch just the tip out of a long lanky stem.....makes it put out side branches....and with some kinda of petunias, makes cutting back unnecessary.
Linda c

You can root the part that you pinch off and get a whole new plant. I cut a couple of inches off, take off the lower leaves, dip in rooting hormone and push the cutting into the dirt. Sort of like several for the price of one! :)

how long does the rooting hormone work? I've had my bottle for a few years and want to know if it will still work or should I buy a new one?

I've never needed RH to root petunias. I've never needed a mini greenhouse. I've just shoved broken stems in nearby pots, watered well and called it good. =) I think they are very easy to root.

Several of you mentioned the word "cut" and I know that petunias grow bushier if we cut them back after getting leggy. However, I would like to know if any of you "cut" them as opposed to deadheading.......that would mean just cutting the spent bloom off. I have been cutting the spent bloom all the way back to the beginning of the stem but it sounds like maybe I shouldn't be doing that? If I just cut the spent bloom itself, the stem that is left soon yellows and dries up at the end anyway. Does anyone else cut instead of pinch, like I do?

I treat my petunias and calibrachoas just like I would shrubs. I deadhead/pinch/prune/thin to maintain a neat and nicely shaped planting that gets good air movement - particularly around the base & older foliage, important for disease and some insect suppression. It's not unusual for me to remove up to 1/3 of the plant's mass at a time to get a planting looking like I want it to look.

Al

I have found that unless I pinch the ends deadheading doesn't help to keep the plant from getting leggy. Now I tidy up the plant by removing the spent and wilted blossoms without cutting the flower all the way back to the stem. Also pinching the growing end encourages new growth along the stem where the old flowers were removed.