Reassemble starting with the black cover. This pops in pretty easily if you help it bit by bit with a small screwdriver on each side as in picture.

Then put screws back.

Put on both chrome covers and slide them back into place snugly.

Slide on trimmer assembly

Put steel spring clip back on. Screw in steel screws. Pop trimmer closed and put black plastic guard back on.

Congratulations on replacing the batteries despite the manufacturer being environmentally unfriendly, designing a product that makes battery replacement nearly impossible.