Assembling the closet takes three separate operations: installing the base frame, assembling the individual units, and installing the units.
Installing the Base
The platform frame the closet units rest on is built from dimensional lumber. The joints are reinforced with glue and screws. Since there is little dynamic load on the frame, simple joints are all that's needed.
The separate frame is an essential part during assembly. Floors in residential homes are never truly flat and level. A separate frame is much easier to level than an 8ft+ unit with an integrated frame. Once it is installed, the frame creates a level surface the cabinets rest on.
The frame is installed with shims. Attach the frame to the wall with a few screws driven into studs. The frame must be level both left to right and back to front. If it is not, the closet units will lean. The height of the closet will amplify mistakes. At 8ft+ even slight errors will be noticeable. Take your time to level the base perfectly. This will save you time later.
Assembling the Units
The units are built using the cabinet construction techniques listed in the cabinet construction thread. The units are essentially big cabinets. The top, bottom, shelves, and back snap into grooves cut into the sides. Glue and screws create a rock solid assembly.
The units will have to be assembled at the job site. It's very unlikely that they will fit through a standard doorway. If they do, getting around corners in a hallway might be impossible. Windows are your best bet. Make sure there is enough room to tilt the units upright after assembling them on the floor. The maximum height of a box is the diagonal of the side pieces, not its height. The separate base frame might create that extra space you need.
When building boxes this large it is absolutely essential that all pieces and corners are square. Slight errors can translate into a 0.5-1" gap 8 ft across on the other end. The practice of using a single, solid back piece helps square the whole assembly. If the back is square and the side pieces fit without gaps, the box will be square and true.
Assembling the box can be challenging. Few woodworkers have clamps long enough to reach top to bottom. For an error free installation clamping is required. It's just not possible to push a piece into place with muscle power alone. Screws will do a good job at pulling pieces together, but they are not a sure-fire thing. On paint grade cabinets using clamps wherever they fit and screws everywhere else is a good compromise. Any gap can be filled with wood putty or paint. If you plan to stain your cabinets, you must use clamps.
I have found that the best order of assembly is to join the sides, top, bottom, and shelves first. Then drop the top into place. The assembly will be almost square without the back in place, if the pieces have square edges and fit tightly. Without the back the entire box is visible during assembly. This makes it possible to spot gaps and glue drips. The large back piece drops into place and is held in place by gravity.
Once the glue has dried and the clamps are off, it's a good idea to secure the shelves to the back with a few screws. This will keep the back of the shelf from sagging under heavy load. The front will be reinforced by the face frame. The screws will also eliminate or reduce any gap between the shelves and the back.
Assembling the Closet
The units rest securely on the base under their own weight. If the boxes are square and the base is level, the units stay in position without fasteners. Move the units into place. Add shim to level units that are not quite plum or straight.
Adding shims between a unit and a wall can help level the cabinet. Be careful not to bow the side of the unit when you drive in the shim. Points that have lateral support, like shelves and the top and bottom of the unit, are good places for installing these shims.
Drive two screws through the back of each unit into a stud. One at the top, and one at the bottom. The screws prevent the units from tipping or moving sideways. If there are gaps between the back of the unit and the wall, you must add shims. The connection must be tight. Otherwise the screws can come lose after a few years of usage. Secure each unit to its neighbor with a few screws driven through the unit's side near the front. This prevents the units from separating. The front is of particular importance, because it will be covered with a one-piece face frame. The frame will split should the cabinets come apart.
Take the time to get it right
Once you are done, test the installation. Push and pull on different pieces and look for movement. Any movement must be eliminated for the closet to reach its maximum live. Moving parts lever joints apart. Any part of the closet should support your full body weight. We are building quality, after all. The face frame area is particularly susceptible, because it lacks lateral support in some places. Drive additional screws into the wall framing wherever needed.
Overall installation can be difficult due to the size and weight of the units. Take care to level the base perfectly, and aligning the unit will be a breeze. Remember, errors in the foundation flow upward.