Go back to your opening lines and choose one thing from them: a word, image, person, object, feeling, action, or physical detail.
Bring it back at the end and ask:
What can I say about this now?
Because the poem has moved since you first wrote that detail, you now have something different to say about it. In the workshop, the pharmacy returns after the poem has moved through ice cream sandwiches, a childhood friend, summer, baseball, and movies:
“I didn’t realize I’d see my childhood.”
At that point, you have developed the starting material through roughly 8–20 lines, returned to something from the opening, and given the poem an ending.
You do not have to see the finished poem before you begin. You only have to know what to do with the line in front of you.