Decide the Type
As mentioned above, since you have opted for a personally written one, you now have many options to explore about what type of vow you want. Sure it will be modern and catchy like the ones we see in movies, but how are you going to add your personal touch to it? You can write something humorous with anecdotes from your relationship (if you know that it will be appreciated), or you can have a really sentimental one to celebrate your love. Then, next step.