Or do I first need to "dispose of my business" on a SCH C and then file as a hobby in subsequent years?
Yes, you may report direct selling like Mary Kay as a hobby, as long as you have no more expenses than you do income.
And, yes, you should dispose of the business you reported on Schedule C. You may report that you stopped doing business at the end of last year.
Be aware, however, that if you report the activity as a hobby this year, the IRS may look back at prior years and disallow any losses you took. If you reported only gains in prior years, this may not be an issue.
For more info, see "When the IRS Classifies Your Business As a Hobby"
The easiest way to find any section of TurboTax is to use the Search box at the top right side of the TurboTax header. Click on the magnifying glass, type in the topic you need ("hobby income, loss"), hit Enter, and click the "jump to" link to go directly to beginning of that topic.