Fill in what you know — leave the rest blank. The more specifics you give it, the more it sounds like you. The output is meant to be a starting draft you can copy down by hand, edit in your own voice, or use as a jumping-off point. It's calibrated to sound real and a little self-deprecating, not like a greeting card.
Read it aloud once. Cross out the lines that don't sound like you and rewrite them. The point isn't to send this letter — it's to break the blank-page paralysis so you can actually write the one that's been sitting in your head.
See some examples → A handful of real love letters I've written my wife over the last decade.