M8 x 40 screw for each corner. Insert T Nuts before M8 screws and do not tighten them now. Just to keep the frame in shape.

Blind L metal brackets for Z axis assembly should be inserted before M8 screws as well.

There are M8 holes for Z axis assembly on the 320mm aluminium extrusions which should be at the back of the XY Frame. Larger hole should be faced to bottom.

Insert two T-nuts for every corner in order to fasten the L brackets on the next step.

Do not forget to insert T Nuts for Y motor holder in back and LCD mount in front.