Choose your victim.
Ideally your victim should be weaker than you, or slower than you, in case you need to run away fast. Your best choice would be to pick an overweight person. Fatter people have a larger surface area to towel whip, thus increasing your chances of contact. Furthermore, they are unlikely to catch up to you if you dash out the changing rooms. Fat people are also fun to towel whip because they have a larger canvas for leaving pink marks.
However if you feel too uncomfortable targetting people of a certain weight demographic, then try going for whichever available bum is closest to you. Women have a very amusing reaction to towel whipping, so you may try this out in the swimming pool area, or wherever it is you may be.
Your last choice for a victim is to towel snap your best friend, but remember your friend is more likely to beat you up afterwards for embarrassing him in front of everyone else. A good measure of judgement to whether they will or will not beat you is to see how hairy they are. Guys with less hair don't seem as aggressive, although this can't be guaranteed.
Select your towel of choice.
Picking the right towel is crucial to making the best towel whip ever. You ideally want a thin, large towel able to cover an entire body. If your towel is too thick or heavy, your towel whip may be slow and ineffective. If your towel is too long or short, you may not be able to achieve the perfect 'whip' stance.
Picking the right colour is also crucial. If you are doing this for group entertainment purposes, you can afford to go with bright red or flowery patterns. Something which will make the towel stand out clearly.
However if you plan to surprise your friend with a towel whip attack, pick a neutral white or blue to blend in with the crowd and seem unsuspicious. When you get near enough, you can unleash your towel whipping abilities on his or her behind!
How to actually towel whip.
The stance is hard to get down at first. Spread your feet out shoulder width apart for balance. Allow your knees to bend for maximum flexibility. Practise shifting your weight from one foot to the other whilst making cliche kung fu noises.
Next, grab your towel and soak with a decent amount of water. This helps you get the nice "wapak" sound of a wet towel snapping. It is generally a good idea not to make it dripping wet as this could make it heavier and harder to use.
When suitably soaked, using your main hand, spin the towel around vigorously until the towel winds up in a whip like shape. If your towel unravels, it is probably not wet enough.
To whip, you need to flick your arm and hand towards your desired victim in a fast motion. The further the distance you cover, the harder the whip lash will be. Shifting your body weight also helps increase the speed of the towel whip.
Planning your escape.
You never know when the victim will choose to fight back, so make sure you have an escape route planned. Plan a path to go over obstacles as your victim will not be expecting this. For instance, fat people will avoid jumping over benches.
Also, make sure you have suitable running gear. If you're holding things, or wearing flip flops, you're not going to get very far.
You are now ready to towel whip your mates in the changing room.
Caution: Do not bully people with towel whipping. Towel whipping is an art, and should only be used on close friends and family who will (probably) forgive you.
Note: Towel whipping does not always impress the girls.