First, pick a date. The best time will be your regular meeting, as the parents have already cleared that off their calendar. Give them advance notice so if they work full time or need to get additional child care, it can be easily arranged.
Once you have the date, then it is time to plan!
Here are some resources for you to use.
Girl Scout Bridging Ceremonies for All Levels
The Ultimate Daisy to Brownie Bridging Ceremony Guide
Here are some YouTube videos to give you additional ideas: