This position helps to increase the sense of balance, concentration, and strengthens the pelvic and leg muscles. I should note that tree pose engages the knee joints, and can be difficult if you have existing knee problems. As always in yoga, listen to your body and modify the poses as necessary.
Start off by standing on your mat with your legs slightly apart, about hip distance, but parallel. Shift your weight onto one leg and open the other heel to 45-90° then lift your heel, so your toes are touching the ground and your knee is bent (imagine the above image but with the right toes touching the ground).
Bring the sole of the foot onto the inner thigh of the leg that is supporting the weight, keeping the toes down. I frequently use my harm to help with the initial placement of my foot. Then you can raise your hands and join your palms over your head or in front of your chest (as seen above).
It's important to maintain a straight back in this position so try to practice "head over heart, heart over pelvis" while balancing. Speaking of balance, we want to maintain balance in our practice so I recommend doing this pose at twice, once on each side.
Downward- Facing Dog Pose (Adho Muhka Svanasana)