From DIYinfo.org
Hanging Bi-Fold Doors
Hang a bi-fold door in the same way as a conventional door, with the following exceptions.
1 If the pair of doors is not already hinged together, join them with three brass butt hinges.
2 Fix the door to the frame with two parliament hinges. They have projecting knuckles that allow the door to fold right back, clear of the architrave around the opening.
3 Where two pairs of bi-fold doors join together at the centre of the opening, glue and pin a full-height batten measuring about 38mm x 13mm to one door to overlap the closing edge and serve as a door stop. If the meeting edges are stepped, there will be no need for the batten.
4 Fit a mortise latch and handle to the adjacent door of the other pair.
Hanging Multi-fold (concertina type) Doors
Most internal concertina type(multi-fold)doors are designed to hang from the opening between two rooms, possibly where two large doors were originally installed. They may have no floor channel, but simply hang from an overhead track. They take up far less room when open than conventional doors. Before embarking on the job, check that the four edges of the door frame are square (the diagonal measurements should be equal). If they are not, it may be best to call in a carpenter or joiner.
With modern design this simple idea is now being incorporated to take any number of doors and opening combinations. By using an upper and lower track system the opening can be greatly increased, making it very popular for exterior use. These door kits have to be order to suit your opening and are normally supplied with fittings and fitting instructions.
You Will Need
Tools
Steel tape measure; spirit level; straight-edge; pencil; bradawl; drill and twist bits; screwdriver; hacksaw.
Materials
Concertina Type (multi-fold) door kit.
[edit] Method
1 Measure the height and width of the opening, and order the door. The supplier will allow clearance for the track and hinges.
2 Draw a center line along the top and bottom of the opening, ensuring they are vertical inline.
3 Hold the top track on the line, drill and screw it into place. If the top of the jam is lined with timber that is thinner than 19mm you will have to drill and plug the wall above to take the weight of the doors.
4 Fit the nylon rollers to the tops and bottoms of the doors and get a helper to lift the doors in place.
5 Place doors in the open position and slide bottom track into place, ensuring that the doors are vertically inline screw track to floor.
6 Cut any track cover moulds to length (if supplied) and screw them in place.
7 Now fit the stops and catches in place, following the manufacturer's instructions.