If you took all the classes recently, you're half the way there. Training takes place online in the form of videos, and there are group training seminars and workshop days as well, that occur once a year or two. I've been a WMI for just over two years now at Michaels, and have been to both a workshop day and a two-day seminar. Both were not a far travel, I live in northeast CT and they were both in nearby Massachusetts. They were a really nice experience, getting to meet and talk to other WMIs.
Pay is based on the number of students in each of classes you teach. Depending on your store, and whether there is another WMI there will determine some of your class schedule. You can usually chose the days and times you are available, especially if you have another job. In the store I work, they have other classes in the classroom besides Wilton, so I have to get the days that are available. If you do regular demos (some are mandatory, others you need manager approval to do (and get paid the hourly wage for) you can get more students interested. Some managers are better at encouraging classes than others, and that's encouraging, but in any case you need to be prepared to promote your own classes any way you can.
The Courses 1-4 generally have a minimum of 4 students to run, which is adjustable by the store manager when necessary. The kids and project classes (one-time classes) have different minimums and are paid for a flat rate. Once you get hired and trained, you'll have access to the WMI website which is full of this information and lots of resources to help promote your classes.
If you haven't taken the recent Courses, within the past three or four years, you can and should probably retake them with another instructor before you start teaching your own classes--ask your CEM and Wilton supervisor about it. They don't just dump you in before you are ready, but you have to have enthusiasm, confidence and willingness to give it your best effort.
In my area we don't make a living at it, so your local economy will have a lot to do with your class income so don't expect a full time job. Start slow, and watch the videos, read and follow religiously the instructor guide and realize and accept that you are teaching a scientifically developed Method of Cake Decorating, that's designed to be taught, and learned by almost anyone, to great success!
Most of all, enjoy the process!