Oh, it's so easy, just write how much you love your partner and how you will never ever stop loving them as long as you both shall live!!!
Ah words! They are so beautiful the way you can string together nouns, verbs, pronouns, adjectives, adverbs, similes, metaphors, antonyms, synonyms, and so MUCH MORE!!!! There are so many ways you can pair up words to evoke feelings of love and happiness while reciting your vows. It can really make your toes curly and butterflies awaken in your belly!
Alas, everyone does not have the same love affair with words. Actually, not very many people would think of it as a love affair. You may only be concerned with not sounding foolish when reciting your vows, right? This is understandable, however, you can't be afraid of words.
This may sound Disneyish, but look inside of yourself. When writing your vows, don't try to say things that are not uniquely...YOU. It is important to say words that are natural to your vocabulary. For instance, if you never use thus, alas, thine heart - then don't use it now! Of course, that is an extreme example of vocabulary but hopefully you get the point.
Writing your vows is writing the words your heart wishes it could say, however, it must be organized in your brain to come out of your mouth. Remember, the reason why your partner is marrying you, is because they love you, and anything you say during your vows will be beautiful to them as long as they are your words.
Most vows include:
What you have been through together
Promises on how you love and cherish your partner forever
Snippets of poetry or scriptures
However, if you are not a poet don't try to force it. So the one tip... vows are coming from your heart, they are your words, and feelings. So write those words because that's what your partner wants to hear on your wedding day.