Pass the x-carriage between the threaded rods at the top of the printer.
Press the x-carriage onto the LM8UU bearings on the x-axis. Use your fingers and thumb like a clamp, pressing the LM8UU bearings into the carriage. Considerable force is required. DO NOT press down on the carriage without supporting the x-axis guide rods from below - something important will break, causing sadness, anger and/or frustration.
Slide the extruder drive mount onto the top threaded rods of the frame. Secure it with M8 washers and nuts.