So set, before the echoes fade, The fleet foot on the sill of shade, And hold to the low lintel up The still-defended challenge-cup.
In the poem's second-to-last stanza, the athlete's coffin has reached the gravesite. Really. That's what's happening. Let us explain.
The speaker tells the funeral procession (and us readers) to place the athlete in the tomb before the "echoes" of applause fade away and to hold up once more his victory trophy in celebration of the athlete's life.
Those "fleet" feet are the athlete's.
The "sill of shade" refers metaphorically to death—or more specifically, the doorway to death.
A "sill," like a windowsill, can also refer to the base of a doorframe (like the doorway to a tomb for example).
The "sill of shade" echoes the "shady night" from line 13 and continues the metaphor of shade and night representing death.
Line 23 has another vocab-grenade. Don't let it throw you off. A "lintel" is just a small piece of lint. No. That's a lie. We're just checking to see if you're still awake.
The "lintel" in line 23 refers to the top support beam in a doorway.
With that little knowledge-gem in the bag, the image becomes pretty clear. The speaker wants the mourners to hold up the athlete's trophy, the "challenge-cup," at the doorway of the athlete's tomb. The speaker wants to send this guy out in style.