I'm putting together 3.25x10 a/c galvanized dust work

I'm having a little difficulty connecting the straight stacks to the
elbow fittings.

As I have found it takes some manual dexterity to attach the fittings,
I'm wondering if I'm doing it right.

Does the assembed two piece rectangular stack runs go inside the
elbow connectors OR
do the ends of the metal elbow connectors insert inside the
rectangular straight stack ???

Thanks for your help!