What is involved in sizing and designing ducts and ductwork?
You need to know how to calculate the required CFM - cubic feet per minute - that your ductwork will allow.  On a room per room basis you need to determine how much air flow is needed, how large the vent needs to be, and thus how large of a duct you require.  Then you need to determine how large of a trunk you require and at what points you need to reduce it after branching off legs from the trunk.

Finally you need to also know where to position the return air and how large it needs to be, in order to support the output from your HVAC.

I don't know how to calculate those things - I just know that's what you need to know.