Start in Tadasana, then turn your right arm actively outward (so the palm faces away from the side of the torso), bend the elbow, and grip the outside of the right foot. (You can also grab the big toe with the first two fingers and the thumb.)
Inhale, lift the right leg up, keep the knees together and bring the thigh parallel to the floor. As you do this, rotate the right shoulder in such a way that the bent elbow swings around and up, so that it points toward the ceiling.
Reach the left arm straight forward, in front of the torso and parallel to the floor.
For a progression you can grab your foot with both arms.
Don’t worry if you are not there yet, you can use a strap and start slowly walking your hands towards the foot.