Tree is a great poses. It engages your butt in ways that help you feel good and sit better and also help how you stand and walk.
Reclined Tree
Start here. Feel the pose and let it work while stable and supported on the mat with the help of a few pillows.
Get on your back on the mat and use a pillow for your head if that is more comfortable. Extend one leg straight along the mat pushing that heel away from your shoulder. Put a pillow under that knee if that is more comfortable. Point the front of that thigh and the front of your pelvis straight up.
Bring your other knee up and keep that foot on the mat beside your extended straight leg. Let your bent knee fall open to the side. Use a pillow to support your knee. Keep the front of your pelvis pointing straight up. Put the sole of your foot on the inside of your extended leg but not on your knee, and let the outside edge of that foot rest on the mat. If you can, put your foot on the inside of your thigh with your toes near but not on your knee. And rest in this position.
Do the pose with no effort. Just allow support and gravity and time to help you slowly release. It feels good. This is enough.
Engage
When you know the pose and have a feel for it and a feel for yourself in it
Engage your butt on the side of your straight leg to tilt your pelvis back a little and feel as you extend along the front of your hip. Engage your butt on the side of your bent knee to pull that knee a bit closer to the mat and feel as you extend your groin. Hold how you engage both sides of your butt then release slowly. Then do it again. Then do your other side the same way, then do both again.
Lift the foot of your bent leg off the mat and press it firmly against the inside of your straight extended leg, but not on the knee. Then engage your butt both ways, hold, and relax slowly. Then repeat, then do your other side, then do both again.
Seated Tree

Sit on a firm chair of standard height like a dining chair. Shift sideways on the chair and let one leg fall off the chair with that thigh pointing down. Swing your other leg open across the seat of the chair. Then engage your butt as in the reclined pose. Then do your other side, then both again.
Kneeling Tree

This is my favorite variation to teach. It allows you to move more than when reclined but is also stable. With it I can better guide how you align your pelvis and engage your muscles.
Get on one knee on the mat. Have your butt just a little behind your lower knee.
Swing your raised knee out to the side only as far as comfortable. It doesn’t matter how far. Keep that knee directly above your ankle to avoid strain. Then engage your butt as in the reclined pose. Then do your other side, then both again.
Supported Tree
Stand beside a firm chair of standard height, like a dining chair. Face in the same direction as the back of the chair with your feet parallel to the side of the chair. And rest your nearer hand on the back of the chair.
Put your near foot on the seat of the chair. Swing your raised knee out to the side only as far as comfortable. It doesn’t matter how far. Keep that knee directly above your ankle to avoid strain.
This works just like the kneeling pose. Engage your butt, then do your other side, then both again. You can put your foot up on anything, not just a chair. I teach this often because it is convenient.
Standing Tree

Stand on one leg. Put the sole of your other foot on the inside of your straight leg but not on your knee. Keep the knee of your straight leg slightly soft. Rest a hand on the back of a chair or a wall as needed for stability.
Engage your butt on the side of your straight leg to tilt your pelvis back a little and feel as you extend along the front of your hip. Engage your butt on the side of your bent knee to pull that knee out a bit more and feel as you extend your groin. Hold how you engage both sides of your butt for some seconds then release slowly. Then do it again. Then do your other side the same way then do both again.
You can increase the pose by pressing the foot of your bent leg firmly against the inside of your straight leg, but not against your knee.
Arms


Put your hands where it feels good for you. Putting my hands together in dhyana mudra resting on my pubic bone helps me to focus on aligning my pelvis and engaging my butt. You can also try anjali mudra or namaskarasana.
Refine
Tree pose reshapes how we move to increase our comfort. But the standing pose can engage the tfl which leads to discomfort. And the more unstable we feel in the pose, the more we engage the tfl.
We use touch to help avoid the tfl. Put your fingers on the top of the tfl just below the iliac crest and just behind the iliac spine. Press your thighs apart or turn your thigh inward to feel the tfl engage. Engage the pose and put your thumb on your gluteus medius and feel it engage for stability instead of your tfl.
Using the reclined or kneeling positions of the pose can help find your feel. And guiding you helps to refine the pose.
Tree pose gives us a pattern of moving. We engage the butt to extend along the front of the hip and groin. Avoiding the tfl helps us in the pose and throughout the day.
Before
Constant bear makes a nice warm-up.