Start by tightening the 8x M3x6 socket head screws for the Z MGN sleds making sure they are straight.

Tighten the 2x M3x40 socket head cap screws and the M5x8 button head screw into the 2020 extrusion.

Snug the 4x M3 button head screws for the lead screw nuts.

Spin the lead screws together making sure the X 2020 can travel the full Z length without binding. If everything moves smoothly snug down all the screws.