Warning: unless you are a pro, you will need someone to help you with this. Preferably someone who's good at French braiding.
Step 1: Flip your head upside-down while your friend gathers your hair together. They'll want to make sure there's no knots or tangles (it won't look nice if it's noticeable).
Step 2: Have your friend start French braiding your hair from the bottom to the top of your head (wherever you plan to have your bun placed).
Step 3: When the French braid is to your desired spot, hold your hair and carefully flip your head back then tie the remaining hair into a bun.
Voila! A French braid bun. Kind of reminds me of an upside-down/reverse French braid 'do, doesn't it?
Special kudos for my little sister Sus! She did the French braid bun for our little piglet. Not too shabby for her 1st try huh?
Have fun trying this new 'do!