I know of 2 different methods to tie the Thai Wrap Pants.
In the following I will illustrate the different steps with "live" pictures.
1. Wind the trousers at the waist with one hand and draw to the outside.
2. With your other hand pull the fabric at the waist to the body.
3. Fold the excess fabric to the right on your waist.
4. With your free hand tie the ribbon together in the front.
5. Place the fabric over the ribbon and draw to the bottom.
The second method is a little more complex as can be seen from the shop pictures.This method is preferred by Thais. In steps 3 and 4 an extra hand can be quite useful until you get it down by yourself;)
1. Trousers pulled apart as seen in the picture.
2. Use both hands to form a "S"-shaped fold.
3. Fix with one hand holding the 1st S-fold while the other hand forms the 2nd S-fold. This is the most complicated part.
4. Hold the two folds on the body with one hand while the other hand gets the ribbon to the front.
5. Knot the ribbon together in the front
6. Place the fabric over the ribbon and draw it down so that the crease in the double S is preserved
7. Done (in this case, even if not perfect, but the principle should be clear.)
Method 2 is much more complicated, however, depending on the model of the pants it looks more symmetrical. Ultimately a matter of taste:)