How to do a masters proposal? Suppose that I have an idea "not fully developed" of course but a sketch somehow. If I want to convince an advisor by it and let him take me seriously. What should I do?
My field of study is Mathematics. I only got an intuition that might work or not. Should I make sure that the proposal is valid before I talk to my advisor or what should I do exactly? any ideas would be appreciated.

Thanks!
Formalize the problem and identify any trivial cases. 
Figure out where a result similar to your idea would be published and try to find it. Have a list of the places that you looked and where you didn't find your result. This will allow you to tell your professor something along the lines of "Professor, I checked ...noteworthy journals in your exact area... as well as ...famous textbooks in the area of study... And I didn't find this result." 
Know why your idea is intuitively correct. Try to find counterexamples. 
Outline your plan of action. 
Be able to tell your professor why you want to work with him/her. Is it because you two work well together or because he/she is a major person in the field? 
Bonus: State why your result would be (or could be) meaningful.