Hold open the pump body casing and align the pump parts into the grooves supplied inside. This isn't hard and makes sense as you move them into place ... take your time.

Notice in the 2nd and 3rd photos the mop rod (that connects the handle to the pump body). It has another rod inside that attaches to the plunger/pistion to make it go up/down in the cylinder. This needs to connect properly to the top of the plunger/piston AND be properly seated into the pump casing.

Test your assembly by simply placing the 2 pump body casings together - they should fit tightly and the trigger should work the pump smoothly.

Once you are satisfied everything inside fits properly put the pump case back together, careful to line it up so it fits perfectly snug with no gaps and the entire pump assembly is seated and secure. Ttighten up the screws.