Be careful with the extruder during this step, it will slide when you rotate the printer to access screws under the heated bed.

Take 4x M3x8 button head screws (reused from your Original Prusa) and secure the heated bed in the corners. Do not fully tighten the screws yet.

Using the 2x M3x12 screws, secure the y_belt_holder to the bed. Do not fully tighten the screws yet.

Fully tighten all the screws evenly, incrementally and in turn, in this precise orders (do not use excessive force or you might break the spacers):

1st the screws of y_belt_holder.

2nd one of the corner in diagonal.

3rd the other corner in diagonal.

Repeat until all screws are fully tightened.