Warrior 1 is a bent knee standing yoga pose that can be used to strengthen the legs as well as warm up the body.
When I first learned this pose the general instruction was to get the front thigh horizontal while at the same time squaring the hips to the front.
I couldn't do both so I tended to focus on keeping my hips square to the front.
Note that the pose is a lot like a lunge with the main difference being that in warrior 1 the back foot is flat on the floor. Because of this, warrior 1 can be used as a stretch for the back leg. If you are working on front to back splits with the back leg turned out (versus having the back knee pointing down) Warrior 1 could be used as a warm up.
Rather than "just doing the pose" one way that this pose can be used is a way of exploring your body and how you can use it more effectively.
As an example, say you want to get your front thigh horizontal but at the same time also want to get your hips square to the front. How can you go about doing this with minimal effort?
If you stand with one leg forwards and the other back, with both knees straight, you can turn your hips towards the back leg side. You may find that your pelvis lifts higher. Turning towards the front leg side, you may find that you can easily get your pelvis square to the front and from there you may be able to turn it slightly towards your front leg side. In the process your pelvis sinks deeper.
If you do these two actions, turning towards your back leg side, then your front leg side slowly and smoothly, you may find that turning towards the back leg side, because you have to lift your pelvis, you have to work harder, but turning towards your front leg side you can actually relax.
Now the trick then is, how do you relax into the hips square to the front position (or turned to the front leg side position) with the front knee bent?
Before going on, some very basic "alignment" points. As you go deeper into this pose, I'd suggest moving your front foot forwards (or your back foot back) so that the front of your front knee, when viewed from the side, is over the middle of the foot.
Some may suggest having your heels aligned in this pose when viewed from the front. For the sake of hip and knee comfort, I'll suggest here that you vary the side-to-side width of your feet so that your hips (and knees) feel comfortable. In addition, a slightly wider stance can make it easier to stay balanced.
As you get more comfortable in the pose, and if you choose, work at aligning your heels, or even aligning your front heel with the instep of the back foot.
With legs straight, the legs are self bracing. With the front knee bent, you'll have to brace the front leg, in particular the knee and hip to prevent both from moving forwards when you sink your pelvis down.
One way you could do this is by bracing your knee and hip with your foot. Press the forefoot down without allowing your heel to lift.
Another idea is to friction your front foot forwards and your back foot back, with minimum effort. Allow your hips to sink down (or stay down) as you do this action and use it to get your hips square to the front.
And once you get that, make your spine feel long.
As you get closer to the hips square to the front position, you might choose to vary the turn out of your back foot while in warrior 1.
Try turning the foot slightly inwards or outwards so that you foot, knee and hip are all comfortable.
Another option, once your foot is in position is to rotate your leg relative to your foot.
Focusing on your back leg hip, rotate the thigh inwards or outwards so that your hip feels comfortable, then adjust foot position (and thigh rotation relative to your foot) to suit.
With hips square to the front, or not, if your front thigh is horizontal, you'll probably find that you pelvis is tilted forwards. i.e. your pubic bone is moving downwards while your sacrum is lifted.
This is natural, particularly when you have one leg back or extended. Rather than trying to tilt your pelvis back (say by pulling up on your pubic bone), adjust the amount of tilt so that both hips feel comfortable.
If you do want to pull up on your pubic bone, remember, or know that you can adjust the amount of upwards pull.
If your lumbar spine feels uncomfortable, you may find that creating an upwards pull on your pubic bone helps. But prior to doing this, you might want to lift your front ribs first. That is unless you are using your leg and hip muscles to lift your pubic bone, instead of your abdominal muscles.
If you do have lumbar discomfort, another possible way to alleviate this is to make the back bend an active one.
Because of this forward tilt, your lumbar spine will be bent backwards.
One suggestion is to make this an active back bend while in warrior 1. That means activate your spinal erectors.
Your spinal erectors and the smaller muscles that act on the back of the spine, multifidus etc, extend from the sacrum and back of the hip bones up the back of the spine and ribcage.
So when doing an active back bend, work at feeling your sacrum and your lumbar spine. From there, work at making your thoracic spine and cervical spine feel long.
Note that we're working from the ground up here. As you get used to feeling all of these actions you might choose to start with your spine and then focus on your legs. But for now, let's stick with what we have. First feel and control and adjust your leg muscle activations. Then focus on your spine.
You can do all of this with your arms down.
Because the back leg is back in warrior 1, something else that you could try, particularly if you have low back discomfort, is activating the gluteus maximus of your back leg.
Note that one useful landmark and action in this regard is the hip crease. This is the line that connects your ASIC (the point of your hip bone) to your pubic bone. It also seperates your belly from your inner thigh.
With your back leg back, you could focus on opening the hip crease and just as importantly, making it "feel" open. This should cause your gluteus maximus to activate.
As a side note, you could also work at "closing" the front leg hip crease.
If you keep the knee still while doing so, you may find that this action can also be used to help square your hips to the front while at the same time possibly helping you to sink your hips deeper.
When making your thoracic spine feel long, you can assist this by creating space at the front, sides and back of your ribcage. Adjust the space at each side of the pose. Play around with it. Note you could focus on creating space at the back of your ribcage first. Try lifting your back ribs. Then create space at the front.
Also, try the opposite. Practice both, or use the one that feels best.
From there, with thoracic spine long and with arms by your sides, move your shoulders forwards to protract your shoulder blades. Reach your arms forwards while keeping the protraction. Then lift your shoulders.
Your shoulder blades will move slightly inwards as you lift them. That's fine.
Try to maintain a feeling of space between the bottom tips of your shoulder blades even as the top of your shoulder blades (the peaks) move inwards and upwards towards your ears.
Note, the goal here can be to lift your shoulders as high as possible. Then reach your arms up.
Keep your shoulders lifted, straighten your elbows. Make them feel strong. From there, if you choose, work at bringing your palms closer together while keeping shoulders lifted and elbows straight and strong.
If you can touch your palms together, use your shoulder and arm muscles to keep them together.
Note, if you choose to tilt your head back and look up, make the back of your neck feel long and strong as you do so.
The simpler option is to keep your head level and look straight ahead. In this case you can also make your neck feel long. Try drawing your ear holes away from your shoulders. At the same time, try dropping your chin slightly without opening your jaw.
The arm actions are something you can practice while standing or even sitting. Likewise, the neck action, bending your neck back, this is something you can also practice while standing or sitting, in combination with bending the neck forwards, to the side and also twisting it to either side.
As a side note or end note, because the back leg is externally rotated while flexed, you might choose to counterpose warrior 1 with eagle pose, a pose where the hips are flexed and internally rotated.
As for the arms, you might choose to counterpose the warrior 1 arm action by reaching the arms back (but without clasping so that you are using your shoulder muscles to drive the action.)
For detailed lessons for opening (and closing) the hip crease, attend the online hip crease course.