Align the T-Nuts and the y-carriage (see Fig. 1). Make sure that there is one T-Nut in front of the hole and one behind it in each of the y-aluminum extrusions.

Place your z-extrusion over the hole in the y-extrusion and fix it from below with a M8x40mm Hexagon Socket Head Cap Screw.

After tightening, loosen the M8x40mm Hexagon Socket Head Cap Screw just enough so that the profile can still be moved easily in all directions.