Describe the sound of footsteps using onomatopeia?

That all really depends on how you want them to sound and what kind of shoes the person is wearing. You could you "clinking" for example if they are cowboy boots with spurs, if they are heavy boots you might use "banging". It can also be used to describe, for example, something walking on leaves, in which case you might use "crunching" or "crinkling" or "crackling"

Yout could use Words such as "padding", "thumping", "stomping", "booming", "splashing", "bamming", "clanging", "clanking", "squelching", "squishing" or "thudding".

ex: "The old man was so into his own thoughts that he failed to hear the soft padding of footsteps approach him from behind." ; "The banging footsteps neared as the boy lay only half concious."