How to do walking meditation?
This is to be mindful about walking. Walking creates sensations in the body like:

touch of your feet on the ground
the tension of the muscles
sensations due to change of posture
sensations due to fatigue.

Whatever sensation:

If he feels a pleasant feeling,

he understands that it is impermanent;
he understands that it is not to be clung to;
he understands that there is no delight in it.

If he feels a painful feeling,

he understands that it is impermanent;
he understands that it is not to be clung to;
he understands that there is no delight in it.

If he feels a neutral feeling,

he understands that it is impermanent;
he understands that it is not to be clung to;
he understands that there is no delight in it.

If he feels a pleasant feeling, he feels it in a detached manner.
If he feels a painful feeling, he feels it in a detached manner.
If he feels a neutral feeling, he feels it in a detached manner.

Dhātu Vibhaṅga Sutta