First thig to do is clean everything with a mild detergent, and rinse off thoroughly. Ensure everything is thoroughly dry before you try to glue or weld it, it works much better that way! During construction try everything as a dry run first, you do not get a second attempt with solvent weld. Work in a well ventilated area (it stinks), and apply liberally all the way around the pipe / join to be made. When making the joint, quickly twist the items to spread the solvent weld, and ensure a watertight joint.

Top Section construction

Solvent Weld the access cap to the top main fitting, then a 65mm length of 40mm pipe in to the side outlet if plumbing the unit as bypass. Note the orientation of the Top main fitting, as the access cap will fit both ends.