Which part is the tail?
The piece circled in the image is known as the 'tail' of your chair attachment frame. It attaches the frame to your bicycle.
Aligning the tail
Bring the tail piece to the head tube of your bike (the head tube of your bicycle should sit between the tail. Next, ensure the holes of the tail are at the center (or as close to the center as possible) of your top and bottom tubes. This is important to ensure weight will be properly supported.
*If you have previously marked your bicycle during the initial 'measurements' stage, align the tail piece to your markings to ensure they line up. If they are even slightly off, be sure to make new markings so that you're drilling in the correct area in the next steps.
Once your tail is aligned with the center of the top and bottom tubes of your bicycle, take a marker and fill in the holes of the tail.
(This will allow you to see exactly where you'll drill in the next step.)
*It's helpful to have someone hold the tail piece in place while you do this to ensure it doesn't move around.
Time to drill
Using a 3/8 drill bit, you'll drill into the marked dots on your top and bottom bicycle tubes.
*Important: Keep your bicycle steady during this stage to avoid incorrectly drilling away from your markings, or at an angle. The drill bit needs to go straight through, not angled. Otherwise, the bolt in the next stage may not pass through correctly.
Bolt it in place
Using the bolts (included) that came with your tail piece, insert them through the holes you've just drilled. Next, using the washers and nuts that the bolts came with, tighten everything in place.
*Important: Ensure everything is snug and tight as the tail helps support the weight of the passenger.