Tadasana is a base asana for all standing asanas. If you learn how to properly stand in Tadasana, and understand the basis of Tadasana alignment, you can use this knowledge to align your whole practice.
So, let’s start from the base up:
The feet are firmly grounded and the weight distributed between three major points on your foot; on the heel, just under the thumb and just under your little toe. The weight should be distributed evenly on your foot, not using the toes. The thumbs are touching, but keep your heels a few centimeters apart. In other words align the outside line of your foot with the line of your mat. This is a more neutral position for your hips.
Engage your tights, but don’t overdo it. Relax your gluteus muscles (your buttocks), or don’t squeeze them voluntary. I often see students squeeze their glutes when I say ‘engage your mula bandha’. However, mula bandha has nothing to do with your buttocks:).
Pelvis should be in a neutral position. Usually, most students (especially more flexible ones) will tilt their pelvis forward. To avoid this, you just have to engage your lower belly (it’s a partial Uddiyana bandha engagement). Imagine you are trying to pull your navel closer to your pubic bone (again, don’t overdo it!). This engages your core and gives your lower back a much needed rest.
Place your hands on your lower ribcage and pull the lower ribs in. This is a common misalignment in most students that are open in their shoulders and chest – instead of being straight and neutral they actually backbend. If you do this, when you pull your lower ribs in you should feel a release in the middle part of your back.
Roll your shoulders up and down to extend your neck, but don’t pull them to the back too much (again, if you are more flexible in your shoulders and chest this is a usual mistake). You should feel your shoulder blades away from each other. If you are closed in your shoulders and they tend to go forward, pull them back a bit and imagine you are making more space between your collar bones.
Imagine the top of your head is trying to touch the ceiling.
Use your breath. Ground more with every exhale, extend more with every inhale.
Every body is different. There are no general rules of alignment that can be applied to everyone. So, see if you can learn about your body and practice with awareness. And when in doubt, ask your teacher. We are here for you!:)