First of all make sure that your suggestion isn't posted already.
If it isn't, make a thread with the subject "Suggestion: [Your suggestion]".
The structure of the suggestion should be like the following:
Name:
[Your MeC suggestion-name]
Describtion:
[A small describtion about your suggestion, where you should write, what your suggestion is for]
Recipe:
[The recipe for your suggestion. Minecraft smilies will be included soon]