Steps
Write a page of standard prose, as fast as you can, about how you felt the first time you saw your loved one, how you felt the first time you knew you were in love, and how you feel right now about being together. These three moments in time will create the structure of your poem.
Replace any weak verbs with stronger verbs and any pronouns with proper nouns. Words depicting the 5 senses work well for love poems.
Reread your passage and pick a metaphor to tie the three moments together. Choosing a metaphor is the most fun part so be wild with it. An opening flower is a tried and true metaphor for love, but a cloud that looks like a heart might work even better.
Rewrite your passage using the metaphor to describe the three moments.
Read your page aloud, change things that sounds "off" to you. Make notes where you feel there's a pause in the flow.
Write the poem, putting a line breaks where you made the notes.
Whether you type or write the poem consider framing your poem. Your loved one may want to keep the poem as a memento!
Read the poem aloud to the person you love, or present it as a gift to them.
Let your poem come from your heart, not your mind or soul, but how you feel. As you think about how you're feeling deep down inside your words will come out through your finger tips, slowly your words will unwind and become something beautiful on a piece of paper. Express your feelings don't hold them in.

Tips
Who knows — you may just write the "Greatest Poem Ever". Just remember your poem is for the one you love. So make it personal and sensual.
Think about your 'love', how you feel when you are together or apart, what you miss most when you're apart, how you feel when you reunite. Write your thoughts and feelings. Poetry comes from your heart — your thoughts will create a love poem based on your feelings.
There's no need to be intimidated by rhyme schemes. Remember, poetry doesn't have to rhyme. Former Poets Laureate Robert Pinsky, Louise Gluck, Billy Collins and Ted Kooser all write poetry that does not rhyme.
The best writing advice is simple: omit needless words. One strong verb steamrolls any three weak ones.
Poetry and almost all artful prose is about how the words reveal your feelings. Read your writings aloud to yourself to feel what your words are saying. If they stir emotion in you, they will for the person you love.
Make it personal. If your love is named Kim or Vern try to use it. Don't use clichés', find something unique or special in your relationship and write about that. Your poem should be a reflection of the love you both share.
One useful tip for any kind of poem is to "write it twice: first with the heart, then with your brain". Express your feelings without sounding corny.
Read poems for ideas but do not copy them. It will get you nowhere, and your loved one may find out.
It's fine to quote an existing poem. Just include the original author's name and say something like, "I thought of you when I read this."
Some people are more poetic than others when it comes to expressing emotions, don't give yourself a report card, your poem doesn't need to be perfect just yours. A short, simple, romantic phrase or rhyme from your heart can be put in with a small gift to give your loved one. E.G. "I hope this gift will let you see ...how much, my love, you mean to me..."
A poem that can be written in prose isn't a poem, so don't follow the method of writing a huge block of prose, cleaning it up, and then cutting it arbitrarily into lines. Learn to write real poetry in meter--that will show your lover real dedication and effort.
Recognize that this is only ONE way of writing a poem. Google "love poem" for a million others. The issue here is not how good a poem it is, it's how much emotion it evokes in your partner. You don't have to stick to the night you met. Try other powerful moments, for example the evening you two walked together just before the birth of your first child, the time you two got caught in the rain, clutching his/her hand on the roller coaster, etc. etc.
The sonnet may be the best format for a love poem. Don't cheat on the syllable rule either - it will ruin the rhythm. Look to Shakespeare for help.
If you want to rhyme, there are great rhyming dictionaries on the internet. Try to stay away from one syllable words and try making every other line rhyme.
Golden Rule: If you are sincere, your partner will love it. Guaranteed.
If you really like them you don't need a poem, a boy/girl would love you if you just told them how you felt about them. Be honest!!