Ustrasana, Camel Pose, is a 3* on the scale of difficulty. While Iyengar's instructions for the asana are fairly straightforward, some additional suggestions may supplement his instructions to make the pose more accessible to all.
To prepare for the full pose, Iyengar instructs the practitioner to begin with the tops of the toes on the floor. If, when you take the backbend, you have trouble reaching the soles of your feet, try beginning with your toes tucked under you. Tucking your toes raises your ankles up higher and makes them easier to reach. If you still can't reach your heels, place blocks directly outside your ankles and place your hands on those instead.
To come into the pose, Iyengar advises you to reach your right hand down to your right heel, then your left hand down to your left heel. I much prefer to release both hands down to my heels at once. This way you avoid impinging either side of the lumbar spine as you release into the backbend. In order to release both hands down to the heels at once, prepare by first rolling your shoulders back as far as possible, striving to draw your upper body parallel to the ground.
Once in the backbend, Iyengar instructs you to, "Throw the head back." This isn't comfortable for all practitioners. If dropping your head completely back feels unnatural, you can keep your neck in a more neutral position. Just ensure that you're not flexing or tensing your neck muscles.
Then, Iyengar advises you to "contract the buttocks" in order to help ease your hip points more in line with your knees and facilitate a deeper stretch in the dorsal (middle) and coccyx (tailbone) area of the spine. I find this instruction helpful, but some practitioners complain that contracting their butt muscles creates a feeling of impingement in their lumbar spine. Listen to your body and do what feels right.
To move out of Ustrasana, Iyengar again instructs you to "release the hands one by one." As with the entry into the pose, I prefer to release both hands at the same time, draw them into prayer at my heart center, then roll up to a kneeling position.
Iyengar indicates that Ustrasana benefits "People with drooping shoulders and hunched backs", as well as the elderly and even people with spinal injury. I would add that this pose is extremely beneficial to pregnant women, especially as they reach their third trimester. During the third trimester, the expecting mother is likely to feel a shortness of breath as her diaphragm shifts up to create more space for the growing uterus. Ustrasana helps create a sense of space in the chest that can be quite a relief. To modify Ustrasana for pregnancy, place two blocks at their highest height in between your ankles. Then, place a flat bolster on top of the blocks. Roll your shoulders down your spine as much as possible, then place your hands on the bolster behind you. This modification helps emphasize the shoulder opening within the pose rather than the backbend, since intense backbending is not advisable for pregnant women.