This one way valve can be used one of two ways..
With the mouth piece to blow air into a pipe which at the same time restricts inhalation. See figure.1
With the duplicate fitting fitted over the mouth piece to connect two pipes, allowing water flow one way. See figure.2
The two pics (figure 1 and 2) should explain assembly well enough.
This design has been optimised for 3D printing, no support is needed, parts printed separately can be assembled by (click assembly) fitting them together. For the best result : print at 100% infill at any layer height you like, i went 0.2mm. (the full infill will stop leaking possibilities.) I highly recommend using glue on the outer parts after assembly if you were to use it with water and pipes, just to ensure a stronger part as the pressure of water flow might dissasemble fitting from mouth piece.
I included two different sizes for the fittings, for different hose sizes: 3/4 and 5/8 of an inch. The fitting's openings become narrower as the pipe or hose gets inserted deeper, allowing a tight fit. Please use a BB pellet or any small ball you can find to use within the valve. The pellet gets caught by the grid when blown on the valve, allowing flow. But when sucked upon, the pellet will reverse into the opening blocking flow.
Enjoy whatever you use it for!