Assembly of the National hive’s super or brood box can be confusing from the instructions given. The video below shows the correct way of assembling the parts. It is most important to check before gluing and nailing that the frames will be at the correct level in the box. For a bottom bee space hive the top of the frame’s top bar must be flush with the top of the side walls, for a top bee space hive there must be a bee space between the top of the frame’s top bar and the top of the side wall. The frame ends do not rest on the wood of the hive, so when measuring, remember to include an allowance for the castellated spacers or runner which will be attached to the inner walls to support the ends of the frame. It is a good idea to have a frame top bar to hand when assembling the hive body and use this to double check before final assembly.
And on the Highfield Bees’ website are easy to follow instructions for the brood box, super and roof.
Pembrokeshire Beekeepers' Association 
Hi. Nice video.
Two quick question:
Are the top of frames supposed to be flush with top of super/brood box?
What is the gap under the frames supposed to be?
When I do my maths: a super is 150mm high, a frame is 140mm high, that means when top of frame is flush with top of super, there is a gap of 10mm at the bottom
Hi Shane. You are quite correct. The hives are what we call bottom bee space hives, which means they have a space under the frames between 6mm and 9mm to allow the bees free movement. The frames are flush with the top of the brood box and supers. Some hives are top bee space where the reverse applies and you have the beespace at the top. You must ensure that your hive is either one or the other as if you mix top and bottom space components you will squash your bees! In the UK the bottom bee space National hive is probably the most common type.
Regards
Paul