In a CoreXY setup both X and Y stepper move together. Technically the top left stepper is the X and the top right stepper is the Y

Start with taking your X motor and slide the timing gear on to the stepper shaft making sure the set/grub screw when tightened will be on the flat side of the stepper shaft.

Note the timing gear orientation. We will need to make height adjustments after the belts are on

Tighten the grub screw

Slide in 3x M3x35mm socket head screws and 3x 29.2mm motor standoffs. Note the front stepper mounting hole that is too large for a M3

Thread in the 3x M3x35mm screws into the stepper but not tight. Note the orientation of the stepper. I noticed this after I assembled this part but the wire plug should towards the middle of the printer. I had to rotate it 90 clockwise from the picture