Push the blue sliding bush back, insert the blue tube into the yellow tube.
The blue tube pushes the red ball out while the pink and brown sticks open the liquid path between the two tubes through the holes of the jade spring bushings.
Release the blue sliding bush so that the red balls lock the blue tube.
To disconnect the coupler, push the blue bush back, withdraw the blue tube then release the blue bush to keep the red balls in their holes.
Two black O-rings pressed by two blue springs and black gasket prevent fluid from leaking.

STEP files of this video:
https://www.mediafire.com/file/leo8e6...
Inventor files of this video:
https://www.mediafire.com/file/i181qd...