Assemble the first HOL as per the instructions, with one change: wait until later to attach the stoppers to the lid. Use pine strips to fill short gaps between the corners, and screw castors into strips.
Using a jigsaw, remove inner wood from front and one end. Sand edges. Remove end panel, cut mesh to fit and staple on. Attach pine edging with panel pins. Screw panel back into place.
Cut 14mm pine strip and use four L-shaped brackets to create door, then cut mesh to fit and staple onto inside. Attach door to HOL using three hinges. Screw in two thumb turns.
Cut two pieces of hardboard to fit floor, and one piece of lino. Attach lino to top layer of hardboard with staple gun. Floor will be removable for cleaning/replacement of hardboard if necessary.
Unpack second HOL and saw base panel in half lengthways to create stairway. Cut and glue on 14mm pine strip using No More Nails or similar adhesive to make edge. Cut and glue on 5mm pine strip to make steps, and another piece underneath the top. Then screw corner hinges into top of end panel and top of stairway. Attach small closed hook to base of edge pine. This will allow stairway to be raised (using a piece of string) to prevent access to top level.
Attach three out of four stoppers to lid, omitting the corner where the stairway is attached, and place lid on HOL. Jigsaw out an opening to allow access to second floor (it’s best if you switch the lid round and do the cutting away from the stairway to avoid cutting into it). Sand edges and attach pine edging with panel pins.
Making the second level
Assemble second HOL (apart from the base, which you’ve already used for the stairway) and saw down the bottom corners so it sits flush on top of the first one. Cut two hardboard panels to make the floor of second level. Double up to make these thicker. Glue them in position, then lift off the top HOL. Cut lino to fit the hardboard panels and attach with staple gun.
Using the jigsaw, remove inner wood from front and one end as with basement level, then take HOL apart, but this time make a door for the end and attach mesh (and edging) directly to the front. Attach door using 1-2 hinges, and screw in two thumb turns.
Use hardboard and 5mm pine strip to create bedroom box measuring approx 43cmx37cm. Cut out doorway. Take litter 42cm long litter tray and use hacksaw to remove one long edge. Insert tray into bedroom.