Finding the Four Corners of the Feet
Look at the bottoms of your feet (sit down to do this!). Press into either side of your heel bones. These points could be considered the back corners of your feet. The front corners are below your big toe, on the fleshy big toe mound, and below the baby toes. If you place your fingers on the heel corners and connect the dots up to the toe-mound corners, you'll feel a lengthening in the bottoms of your feet.
Tadasana -- Mountain Pose
Stand with your feet together. Look down at your feet and notice their length. Keep in mind the four corners and root the heel corners of the right foot into the floor. Lift the front of the foot off the floor and lengthen the front corners forward and place that part of your foot on the floor. Did you see the right foot get longer than the left?
Do the same on the left. Now spread your toes.
Draw both sides of your kneecaps up so that your quadraceps muscles ( front thigh muscles) engage. Take the flesh of your buttocks toward the floor. Now bring your awareness back down into your feet and notice how your weight is distributed there. Can you balance it over all four corners?
There won't necessarily be a static place to balance the four corners, but in tadasana we're always seeking it out. Imagine little roots descending through those four corners, keeping you connected to the Earth. Notice as the corners descend what happens to your pelvis, your spine, your chest. Did anything happen to your breath?
These are all things to notice and explore over time and with practice. Also, you can take the awareness of your four corners into the Home Practice and discover what happens to them when you do Trikonasana (Triangle Pose) or Virabadhrasana II (Warrior Two).
The great thing, though, about Mountain Pose is that you can practice it anywhere. Grocery lines, bank lines, your kid's soccer game. This pose doesn't need to be restricted to the mat!