THIS STEP IS NOT OPTIONAL. If you don't do it you might be unable to finish the assembly of your Bear extruder.

Try to insert the belt from your original Prusa in the Bear x_carriage. Use a slotted screwdriver to make sure the belt goes fully inside. If you have trouble inserting the belt, it might be due to the following issues:

Your prints are over-extruded. When you print functional parts you MUST calibrate your extrusion multiplier. This is not optional, every spool is different and will melt differently. Check our calibration guide: Extrusion multiplier and filament diameter

You are not using a genuine Gates 2GT belt or your belt profile is not corresponding to 2GT.

You are using a different filament than plain PETG and the shrinkage coefficient is different. In this case, you can compensate for shrinkage within your slicer.