“Standing straight on the left leg, bend the right leg and place the right foot on the root of the left thigh. Stand thus like a tree on the ground. This is called vriksha-asana.”

1 Stand with the feet together and the arms by your sides (see the tad-asana).
 
2 Bend the right leg at the knee, raise the right thigh and bring the sole of the right foot as high up the inside of the left thigh as possible.
 
3 Balancing on the left foot, raise both arms over the head keeping the elbows unbent and joining the palms together. Hold the posture while breathing gently through the nostrils for about 10 complete breaths.
 
4 Lower the arms and right leg and return to the tad-asana, standing position with feet together and arms at the sides. Pause for a few moments and repeat on the opposite leg.

The challenge of the vriksha-asana is maintaining balance on one leg. Poor balance is often the result of a restless mind or distracted attention. Regular practice of this posture will help focus the mind and cultivate concentration (dharana).
When practicing vriksha-asana it may help to imagine or picture a tree in the mind and apply the following technique: Imagine that the foot you are balanced on is the root of the tree and the leg is the trunk. Continue by imagining the head and outstretched arms as the branches and leaves of the tree. You may be unsteady for a while and find the body swaying back and forth, but don’t break the concentration. Like a tree bending in the wind and yet remaining upright, the body can maintain balance.

Aim to achieve the “rootedness” and firmness of a tree. Regular practice of the vriksha-asana improves concentration, balance and coordination. Because the weight of the entire body is balanced on one foot, the muscles of that leg are strengthened and toned as well.

As you advance in this posture and are able to remain standing for more than a few moments, try closing the eyes and maintaining your balance. Source