The short answer is:

•  be a registered Scouter,
• be qualified and
• "complete the form."  http://www.scouting.org/filest…../34405.pdf

Realistically, you want to have either prior knowledge or an interest in the topic, and the desire to teach/mentor the Scouts whom you council.  If you are going to be unit-specific, check with the unit about their needs.  A Troop should have a list of qualified MB councelors to whom they refer their Scouts.  If you have a skill that is not covered by an existing councelor, sign up for that one.  And oc course, everyone wants MB councelors for the Eagle-required list.

 http://www.Scouting.org has plenty of information about being a councelor and there are other resouces as well.  If this is a direction you want to take, look at the requirements of the badge(s) you want to sign up for and see if there is anyhting you are uncomfortable/unfamiliar with.  Then learn about that part of the badge to get over your nervousness.  As you know, the PTC'ers are more than happy to help if you have any questions.  If you have a specific MB to teach, one of us may have taught it and have materials you can use/adapt.

On the App there is a place to check whether you want to work with a specific unit or all units, so that is how Districts/Councils get their recruits. 

By the way, thanks for the reminder – I'm MB councelor for a dozen or so and need to put in my annual renewal.