First, put the towel around you in such a way that two of its ends are pointing away from you while the other two ends are falling down from behind you. The towel should be half around your waist, with you holding it from the top two corners in such a way that it is hanging on your lower back and the rest is falling up to your knees, or even ankles if the towel is big.
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Then, stretch the end you are holding in your right hand towards your left hand and the end you are holding in your left hand towards your right hand. You are basically making an ‘X’ with your arms now, and the end at the right is now on the left, while the end on the left is now on the right.
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Now, stretch these ends to the limit where it tightens your waist. Make sure it is tight, or the towel is going to fall and create a lot of problems for you if you are planning to walk in a corridor. In fact, it will be suitable for you to wear at least something underneath so that in case you fail to tie your towel properly for the first time, you can at least avoid the embarrassment.
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Now, you are almost there. All you have to do is, keep stretching both the ends and put one end on the towel and roll it down. That way it will get tight from one side and will not fall. Do the same with the second end as well and when they both have been rolled down, use your hands to tighten your towel in such a way that it is comfortable for you.
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If you want your towel to be a little short, either buy a smaller towel or roll it as much as you can. However, if you want it to be long, you will need a bigger towel, and try to roll it less when you are tying it around your waist.
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