Tanning oil accelerates the tanning (or, if you are unfortunate, the burning) process. Most tanning oils have ingredients that slightly protect against sunburn as well--unless you already have a good tan, you should always use an oil that states an SPF number on the label. With oils, these numbers are usually very low, however, so do not rely on these in the way you would a sunscreen.
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Place a towel down to protect whatever you are going to be sitting on (if you will be sitting part of the time) as you apply the oil.
Pour a quarter-sized amount of tanning oil into the palm of one hand. Rub it gently over your other palm.
Bend over and apply the oil to the tops of your feet (unless your feet will be covered by shoes of course). Move your hands up to your ankles, spreading whatever oil is left still over your ankles and calves. Pour another quarter-sized amount of oil into your hands. Do not worry about applying too much--it is better to get too much than too little on your skin before going out to tan.
Work your hands up your legs, applying the oil to every area the sun will hit, including the backs of the knees. Spread the oil all the way up to the bikini line. You will probably need to pour more oil at this point.
Rub the oil all over your belly and lower back. If you do not have someone present to rub oil on your upper and middle back, tilt the bottle behind your neck and let a small stream of the oil run over your shoulder blades. Reach behind you, as best you can to rub it in.
Finish by applying oil to your upper chest, your neck and your face, if you do not break out easily or are using an oil that states it will not cause breakouts (always avoid the eye area, no matter what kind you use). Allow the oil to seep into your skin for a few minutes before you lie down to tan.
Reapply tanning oil every two to four hours, or more frequently if you are sweating heavily or if you immerse in water. You may also need to reapply it if you have been lying on one side, and switch to your other side, as some oil may have worn off onto the towel or you may have sweated some of it off.