First, convert the Bugaboo Donkey 6 from single mode to double mode:
1. Remove the side bag.
2. Unclip the three latches on the front, back, and handlebar.
3. Press the button in the middle of the chassis and slide the pushchair open to the right.
4. Snap all the clips back closed.
Finally, put the sibling seat onto the chassis. That's it — you're ready to go.