"If" demonstrates the message to "sieze the day" through contrast and word play.
For example, in lines 3-7, Kipling writes:
If you can trust yourself when all men doubt you,
But make allowance for their doubting too:
If you can wait and not be tired by waiting,
Or, being lied about, don't deal in lies,
Or being hated don't give way to hating
These lines all encourage completing a task opposite the manner of the rest of society. When others lie, the poem encourages, don't drop to that level and lie too. The concepts in each line demonstrate contrast. Each assertion encourages the reader to take the moral high road and do what is difficult even though it is tough to do so. These lines illustrate what it takes to sieze the day. This is the foundation.
By the last stanza, the speaker, an obvious father figure, encourages his audience to consider the result of laying the above foundation. This result will be the fruit for having done the aforementioned work:
If you can talk with crowds and keep your virtue,
Or walk with Kings---nor lose the common touch,
If neither foes nor loving friends can hurt you,
If all men count with you, but none too much:
If you can fill the unforgiving minute
With sixty seconds' worth of distance run,
Yours is the Earth and everything that's in it,
And---which is more---you'll be a Man, my son!
Having given every possible morally correct effort to living and engaging with people, the son can have everything: the world, material wealth, and reputation.