Approaching supervisor for master's thesis when I have ideas but nothing concrete?
Is this supervisor someone you know/in your department? This makes a  difference in your approach.
If this supervisor is from another school, and you REALLY want to work with them, go see who he has supervised in the past and reach out to them to get a feel for how that professor might respond. 


2a. An alternative to this is to call the departments graduate studies unit, convey your interest, and ask them how you might go about talking with a particular professor. Every university tends to have a prospective students section where you'll find an email for your questions to go to. 


If you are sending an email, prior to you getting into a bunch of details, it would probably save you some time to ask them if they are considering accepting graduate students for the 20xx-20yy year, and if they say yes, express your interest. 
Really make sure that you contact previous students to see how that professor is as a supervisor. This isn't done enough but it is so important to know what you're getting into.