In the Forums under spell suggestions, write your spell; if your spell has been considered by the council, it will be added to SoM. Hope this helps! Good luck! You could also send your spell to someone who is a council and have them decide if it is to be posted.
My opinion is that a spell that you create works better for you than someone elses. Not saying that theirs wont work. But one that you create has your energy, your emotion ect. Also I have taking someone else's at times and tailored it to fit my situation which also works. When you create your own spell it also gives you a feeling of accomplishment. It builds your self confidence which also will help when you cast the spell. Just keep it simple to start with and have fun with it.