fairly strenuous, it is not a posture for the faint of heart (literally, it is not great for anyone with heart problems or high blood pressure) but is a great all rounder for the body: standing, backward bending, balancing. the warrior tones the ankles and knees, stretches the front hip and thigh and opens up the chest and lungs.
basic instructions would go something like this (there are many ways to get there, this is an example of one):
♥ stand with your feet about a leg's length apart, feet parallel
♥ turn right foot out by 90 degrees
♥ turn left foot in by 45 degrees
♥ square hips in the direction of the right foot
♥ bend right leg to 90 degrees
♥ inhale arms up over your head
... et voila, warrior one!
but, of course, this is just the beginning, there is quite a bit of fine tuning of the pose to be done. first of all, what does it really mean to SQUARE YOUR HIPS? the hips are square when they are symmetrical, both horizontally and vertically. (that's right, i thought i'd throw in a bit of geometry.) in other words, one hip should not be lower or higher than the other (horizontal symmetry), and it should not be in front or behind the other either (vertical symmetry). this is the ideal to work towards.
in warrior one, squaring the hips means that (based on the above instructions) the left hip is moving subtly forward and the right hip backwards. this might affect the position of the BACK FOOT if the hips are not very open.
if the hips are not very open, square hips and back foot at 45 degrees with the heel on the ground may be tricky. the good news is that there are some variations available (there are always variations available!): you can either place a support under the heel or lift the heel completely and point foot forward as in a high lunge. the second option also makes the hip squaring exercise easier.
quite possibly the most important alignment to watch in this pose is the FRONT KNEE: it has a tendency to roll inwards and, as a result, all sorts of bits and bobs in the knee get thrown off balance and exposed to injury. so there are two things to bear in mind: first, the knee should be directly above the ankle and never further forward than that. second, the knee should be pointing forward over the second and third toe. stick with these two principles and your knees will be fine.
this is getting a little bit long now, sorry, but i can't skip a final point on the LOWER BACK. with the front knee bent and the other leg extended back, the pelvis likes to tilt forward and in turn cause compression the lower back (painful and generally bad for the back), particularly if the front hip of the back leg is tight (hello runners, cyclists, everyone). the action required here is to lift the pelvis forward and up and lengthen the tail bone down towards the floor. a common move in yoga and an incredibly useful one.
ok, all tweaks in place, it is time to stretch up, open the chest wide and feel the power of the warrior (and yourself)!
p.s. you can find a brilliant anatomical study of the pose here, scroll down to virabhadrasana I.