find the joists and mark where you want the hatch to go then you`l need to cut it out. in the attic you`ll need to put wooden straps across the two open sides of the hole where there is no joists place an inch back from the sides of your cut to allow flushing. use 3" by 3" for these straps . then attach 4 lengths of 2" by 1" around the hole for the hatch cover to rest on . measure the opening an cut out the piece for your cover , then face up round the hole with 3" skirting or whatever this should suffice. I`m no carpenter but I`ve spent my working life alongside them(i`m a decorator) but its a simple enough job just be aware of the obvious ,like live wires an fallin through the ceiling an shit...good luck