In warrior 2 (or virabhadrasana 2) you stand with your legs spread apart. and your pelvis facing the front. One foot is turned in slightly while the other foot is turned out 90 degrees. The knee of the turned out foot is ideally bent 90 degrees with the thigh level and shin vertical. The other knee is straight.
The pelvis can be level from left to right. However, what is more important is that your hips feel comfortable. The spine is vertical.
The arms reach out from to the sides you look in the direction of your bent knee leg.
Because you are looking at one hand in warrior 2 it is easy to drop your awareness of the back arm. And so you have to focus on feeling your back arm. Try to pull your elbow and wrist away from your shoulder. Make your elbow straight. Reach with your finger tips and pull your shoulder blade away from your spine. Reach that arm backwards (away from the back of your head and away from your "back" shoulder.)
To get into this standing yoga pose with, step your feet about a leg width apart. This is a starting distance. You'll probably find that you actually need to go slightly wider.
Turn one foot out 90 degrees to your pelvis. Say your left foot. Turn your right foot in slightly. With your knees straight rool your shins out so that your knees point in the same direction as your toes. Align the two bones of each heel with the bones of your legs. Lift your arches and press down through the inside and outide parts of the forefoot. Reach with your toes.
In this position check that your pelvis is level from left to right.
Keep your foot active and bend your left knee . Your knee should be over your foot when viewed from the side. Your knee should also be over your foot when viewed from the front.
From here, if your thigh isn't level (but your shin is vertical) move your right foot away from your left foot enough so that your left thigh is horizontal, parallel to the ground.
You may find it difficult to get your thigh level and your pelvis level in Warrior 2. Work towards it gradually. Gradually work at widening your feet while holding this yoga pose.
When looking to the front hand people tend to lean their spine to that side. Instead, keep your spine vertical. To do that reach back with your back hand as much as you reach forwards with your front hand.
You can straighten your lumbar spine by pulling your lower belly in and rolling your pelvis back. You can lengthen your neck and open your chest by pulling the back of your head back and up. Do this before looking to the side.
Lengthen your ribcage by creating space between each set of ribs. Lift your ribs away from your pelvis. Lift your head away from your ribcage.
To make your arms as long as possible in warrior 2 spread your shoulder blades. (This uses the serratus anterior muscle.) Move them away from your spine.
Make your elbows, wrists and fingers straight. Look towards your leading hand while staying aware of your other hand.
You could also retract your shoulder blades. See scapular awareness for more.
Spread your fingers or have them together but in either case make them feel alive.