Put the bottom part back onto the laptop as shown. Then push it slightly back to its original position.

Pay attention that the bottom piece engages well and evenly.

Take the four screws (2 for each side) and fix the bottom piece.

Take the screw you removed near the battery compartment and screw it in.

Take the battery and put it back into the compartment.

Take the lid of the battery compartment and close it. That's it!