Okay, folks...here's the thing - Catholicism doesn't normally allow or promote "colouring outside the lines", so making your own prayers isn't a normal occurrence.
(And
@Crazyhole , you're a bad boy and need to stay after to clap the erasers. No, I don't care if you will be inhaling particulate matter. The Lord will protect you and if he doesn't then you must have done something to deserve the punishment. Offer your suffering in silence.)
HOWEVER, in the multi-cultural kumbaya Catholicism of today, you pretty much have no rules. Write a poem, even a haiku. They LOVE those...it's so...ASIAN.
Normally in Protestant prayer (as stated previously), you would address God in repsectful terms, express gratitude for your blessings and make the request. So pretty simple: opening, thanks, request.
Start there with the basics and flesh it in from there. Tack on "in Jesus' name we pray" and you'll be good to go.
What? You expected me to write it *for* you?