He who, from zone to zone, Guides through the boundless sky thy certain flight, In the long way that I must tread alone, Will lead my steps aright.
The speaker begins the final stanza by once again sharing with us the spiritual truths he's realized.
The same spirit, or "Power," that "guides" the waterfowl's "certain flight" (we'll get back to this important little phrase in just a quick second here) will also guide the speaker.
There is a "long way" ahead of him, and he must "tread" it alone.
Don't get too weepy, though. Even though he must tread part of his path alone, he really doesn't have to because the same power that guides the waterfowl will "lead" his "steps aright."
In this classic example of synechdoche, the speaker seems to mean both "aright" in the sense of the proper path, but also in moral sense—"aright" in the sense of doing the right thing, the opposite of sinfully crooked.
We don't want to read too much into this here, but it is possible to read this whole little section as a mini-allegory about sin. One reason the speaker may need his "steps" led "aright" is because he has strayed from the path of righteousness, because he's made a mistake or done something wrong, and now needs to be steered back in the right direction.
Any way you slice it, folks, the speaker is definitely a new man by the end of the poem.
Remember how before he was talking about how the waterfowl seemed to be floating through a "pathless" sky?
Notice now how the waterfowl is characterized by a "certain flight."
"Certain" here means something like "definite" or "well defined," and it is 1000% the opposite of random, indefinite—basically anything you could describe as "floating."
The speaker's final description of the waterfowl proves just how much he's learned in the course of the poem.
It is a triumphant note to end on, and we can the speaker is now a more hopeful, confident guy.