Just write exactly what you want happen. If you want to make it rhyme, you can (give it a nice resonance when you say it) but it doesn't have to rhyme.
If you are working with any deities, you can include their name it in. For instance a spell can simply be,
"Good fortune, I call aloud to thee,
I ask you now to come to me,
For others I have helped, and by rule of three
Goddess (insert name) please bestow to me,
The good fortune I seek,
So mote it be"
I just made it up on the spot (so may not be perfect) but it's an example of a spell that can be used by someone who have helped out others but need some help and good fortune in return for themselves.
It doesn't have to end with so mote it be, but many practitioners include it especially if they are more inclined to white magic. (I don't necessarily see magic as white or black because I think it is highly subjective so it's a mute point for me).