At your monthly Leaders' Meeting, have the Brownie/Junior level consultant ask all the troops what Try-it/badge they want to do. Usually the oldest troops have the most Try-its/badges and can help make the decision.
Pick a date and time frame. In a 10am to 1pm period, using 20-minute stations, you can comfortably do two easy Try-its or one Junior badge thoroughly and have lunch. You can adjust as needed. Divide the tasks and requirements among your leaders.
Secure a location to hold the workshop. In a large all-purpose room the girls can move around the stations without a leader (there are leaders at every station).
Contact the Council Shop at least 2 weeks in advance and place an order for the Try-its or badges you will need for your workshop. If all the Brownies in your service unit are invited, make sure you order enough. The shop will allow you to have the Try-its/badges on consignment.
On the day of your workshop have someone - it could be an older scout - sit at a registration table to sign in all the girls as they arrive. This list need only be name and troop number. This person will tally the number of participants from each troop. Then each troop leader will buy the correct number of try-its/badges for her girls using a troop check.

Each requirement needs its own station and 1 or 2 leaders to run it. The leaders can decide which requirement they want to be responsible to run. The girls can be divided into groups by the number of stations needed to finish the try-it or badge. The girls move through all stations and feel the accomplishment of completing the work "themselves." This is called a wide game.

This can be great fun for the girls and the leaders, too. The leaders feel like they accomplished a lot since many girls finished a try-it/badge that day and they had the opportunity to work side by side with their fellow leaders. This can be an annual event in your service unit. It works out very well to do in the winter months before sports begin. January seems to be a slower month for scheduled conflicts.
This is a great opportunity for the girls to bring a food bank donation or equivalent. You may not need to charge the girls to attend. If you must pay for the room/facility or purchase supplies, then charge an admission fee.

After the workshop, bring the troop checks and the extra badges to the Council Shop for payment.