It's offered from 12:30pm to 4:30pm every hour. They'll take about 20 kids at one time - during busy season, it does get full quick, so if you can get there early (30 mins before) I would do so and line up.
Each child will receive a paper chef's hat with their name on it, then they'll stand in a line on the podium. 2 chefs will assist the kids. Each child gets a chance to put 1 ingredient into the bowl, then each child will dump 2 cookie dough on the cookie sheet. The cookies then go into the "magic" oven and while they wait, they'll clean off their hands with a wet wipe. Once the cookies are done, the kids get 2 Nestle toll house chocolate chip cookies and the chef's hat. Very exciting for the younger kids. The whole thing takes about 20-30 minutes.
This is my son's favorite thing to do at Epcot that we usually go more than once. While he and I do the cookies, DH takes older DS to Innoventions to play video games. It really works out because older DS doesn't get bored waiting for his brother, and younger DS and I don't really care for video games. It's a great way to get 2 things out of the way at one time.