You can't fit a round peg into a square hole. Except in this poem, where you can fit a "round earth" in the "imagined corners." These corners are like those of a map or a giant room. From the corners of this kind-of-flat, kind-of-round, kind-of-square world, angels appear with long, bright trumpets, the kind you might use to introduce a king. These trumpets make a loud, terrible sound – loud of enough that even dead people can hear it.
All of a sudden, the souls of dead folks are going all over the globe, looking for their bodies. How should we imagine this scene? Are they digging through the earth to find their corpses? Or are the bodies laid out in some organized way? Based on this poem, it seems that the luckiest people are the ones who have not died yet but will still be saved.
By line 9 we realize that the speaker has been imagining the whole scene, and it hasn't happened yet. Just as the angels are about to put those trumpets to their lips, the speaker changes his mind about Judgment Day. He's suddenly standing before God, asking for more time. He practically begs God to teach him "how to repent." The speaker doesn't want Judgment Day to come until he has sufficiently repented his sins; he wants to be in that lucky group of souls who are saved. He wants God to seal an official pardon with His blood.