Step 1 - Acquire bristles from a common broom head. Acquire in small batches to prevent waste.

Step 2 - Place bristles into holder to build up an sufficient amount of bristles to add the the vacuum brush replacement. My rule of thumb is that you should have enough that you have some resistance when moving the bristles within the holder.

Step 3 - Add glue into brush arm socket, superglue has a decent viscosity to properly coat the bristles inside the arm.

Step 4 - Insert bristles into arm socket filled with glue buy holding both the arm and the holder together, and pushing in the bristles.

Step 5 - If the bristles are sufficiently in the socket, and you're happy with the result, move the holder away from the brush arm, but keep the holder on the bristles to prevent frayed pieces from falling off as it dries. Do this carefully to not accidentally remove the bristles. This is done so you don't accidentally glue the holder to the arm. (Repeat steps 1 through 4 for each arm that you need for your replacements, 3 per brush node)

Step 6 - Once your glue has dried (varies depending on the glue used.) Insert the arms into the main brush node, and remove the bristle holder.

If you complete these steps above, you should have yourself a functional replacement brush for your robot vacuum, so you can breath new life into your robot vacuum instead of having to buy a new one, or find the replacement part via a shady/slow manufacturer site.