Enter Urdhva Dandasana from Sirsasana with the elbows under the shoulders and the fingers interlaced behind the head. Squeeze the inner legs together and point the feet. Engage uddiyana bandha (navel lock) and mula bandha (root lock) by drawing the navel upward and inward and lifting the pelvic floor. With an exhalation, slowly lower the legs as a single unit until they are parallel to the ground. Take a subtle tuck of the chin as you allow your hips to move forward in space. Be aware of the curve of your cervical spine as you transition and push into the length of the forearms to support the head.
Keep the legs straight, strong, and tightly together. It may be difficult to sense the angle of the legs from your inverted position. Generally speaking, you will need to pull the thighs in further than you think to bring the legs parallel to the floor. A fair rule of thumb is: if you can't see your feet, you haven't come far enough.
However, it is appropriate to build up to the full flexion of the torso over time if one experiences discomfort in the neck or unsteadiness in the pose with the legs parallel to the floor. If the balance is steady and no discomfort is experienced in the full pose, stay for up to one minute. Once the pose has been mastered, it is a useful exercise to extend the breath and very slowly lift the legs to Sirsasana on the inhales, then lower the legs to Urdhva Dandasana on the exhales 5 to 10 times in a row.
As with any type of inversion, carefully but immediately exit the pose if you experience discomfort in the head and neck. The same goes for numbness or tingling in the limbs. Don't tough it out. Come down, adjust the placement of the head, and try again IF you think you can do so safely.
Questions? Concerns? Leave 'em in the comments and I'll try to help you out.