Brainstorm before you write anything at all. What is your favorite thing that your boyfriend does? It could be something so small you have never even mentioned it to him, like the way he holds your hand. Think about your favorite time the two of you spent together, and recall all of the details.
Start writing. You should have three stanzas (like paragraphs), each with four lines. Each stanza can be a sentence, split into four parts. Don't worry about rhyming; it would take away from the serious tone you're trying to achieve. Pause before each line, because you're writing in pen and can't erase. It's always good to begin by reminding him of that special time you two shared, and write the details you remember. For the second stanza, write what you love about him, even if the things seem little or insignificant. It's always good to end with, "I love you," and this is where you can insert a pet name or something funny only the two of you share. Love poems are allowed to be mushy, but ending with a little humor can make things more comfortable.
These final touches are almost more important than the content. Spray the poem with a perfume that you wear. Apply a few coats of lipstick and kiss the stationary, leaving the print of your kiss on the paper. Let it dry for a minute, and then neatly fold it into thirds, so the perfume is enclosed and your scent will linger longer.
The delivery is the last step. The most important part of the delivery is timing. Be sure to present your boyfriend with his poem when his friends and family are not around, in case he's easily embarrassed. Tell him he can read it later, so he doesn't have to do it in front of you. He'll be gracious that you took his feelings into consideration, and even more so after he reads the poem.