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.