CREDITS: http://yogabycandace.com/blog/2013/11/5/how-to-use-a-yoga-strap

How to use a yoga strap in seated poses
In seated postures like the hamstring stretch shown above, yoga straps can be used around the feet as shown. Arms should be straight, and grab hold wherever you can. The pelvis will naturally tilt forward, bringing the body into alignment and bringing length into the hamstrings. It isn't often that we sit like this, so shoulders might creep up toward the ears with tension. Be mindful of that, and relax the shoulders away from the ears, keeping the spine long and the shoulder blades melting down the back. Sternum is lifted.
The pose above is the beginning of baddha konasana (cobbler's pose). It can be tricky for people with tight hips. To support the hips and low back, use the strap as shown. Loop one end of the strap through the buckle and pull tightly. Set it just around the top of the hip bones, and loop the other side around the outer edges of the feet as shown. This will help the spine to stay long, and the pelvis to tilt forward.
How to use a yoga strap in binds
To go a little deeper in the practice, yoga straps can be used when coming into binds like the one shown above. The full bind is in the second picture, and if the hands don't quite meet, use a strap to get the extra length. Use the strap every time you practice a bind and in time, the flexibility in the hip will come and the chest will open up, allowing the hands to clasp.
How to use a yoga strap in balancing poses
For balancing postures, the strap can be used in poses like extended hand to big toewhen the flexibility isn't as developed. Loop the strap around the foot as shown, and grab hold as close to the foot as you can, keeping the body long and in alignment, and keeping the arm straight. Activate the entire leg and breathe.
How to use a yoga strap for shoulders
This is part of gomukasana (cow face pose), and is a pretty intense stretch for the shoulders and chest. The full expression is shown on the right, and if the hands don't meet, use a strap as shown on the left. Try to keep the spine long, and the chest lifted.
How to use a yoga strap in arm balances
Distance between the arms is super important in poses like downward dog, wheel andcrow (shown above). If the elbows tend to go out toward the side, use a strap. Loop the strap through the buckle and tighten around the arms at shoulder width. Inhale and come into the posture - whether you're trying this with downward dog, wheel or crow pose.
Bonus!!! YOU CAN USE IT AS A MAT CARRIER TOO !!!