In the majority of cases, fitted bathroom units come flat-packed and you need to assemble them yourself.  Most of them are constructed in a similar way, using cam studs and plastic connecting blocks.  So, whether you're installing a run of fitted units or separate bathroom vanity, the principles are the same.  Here is a brief guide to how to install fitted bathroom units.

Begin by laying out all the components on the floor, so you can see if you've got all the necessary pieces, and check you've got the right fittings as well.  Follow the manufacturer's instructions.  Generally, the first thing you have to do is use a hammer to fit plastic connectors into pre-drilled holes along the edges of the panels. 

Next, fit wooden dowels into the relevant holes on the edges of the shelving sections.  Push cam screws into the pre-drilled holes by hand.  Ensure that the open end of the screw thread points to the edge of the shelf or unit section.  The next step is to fit the cam studs to attach to the screws in the corresponding section of the unit.  It might be helpful to use a screwdriver at this point to make sure they are tightly in place.

Put the unit together by matching up the sections and inserting the cam studs into the cam screws and then turning the screws to hold in place.  Once you've assembled the carcass of your unit, it's time to fit the doors.  Most flat-packed units have recessed door hinges, which you can't see when the door is closed.  It should be easy to fit the hinges as the positions will already be marked out.  You will just have to screw the hinges into place.  You can also fix hinge plates to your unit, using the pre-drilled holes.  Check that you've got them the right way up before screwing them down, as some aren't the reversible type.

Attach the doors by attaching the hinges to the hinge plates, before starting to fit the sink section.  Generally, the sink section is screwed onto the plastic connection blocks at the edge of the unit.  You put screws through the blocks into a chipboard section of the sink section.  Of course, the precise design of units varies, so be sure to check the manufacturer's directions.  At this point, many units have a drawer front that is attached using connecting blocks above the doors.

Now it's time to work on the back of the carcass to prepare it to be fixed to the wall.  Many manufacturers provide L-shaped brackets for this, and you will need to screw them into place along the back edge.  To attach the door handles, you'll need to use a drill to drill through the marked holes.  It's a good idea to hold a block of wood on the other side of the door to stop the door from splitting when you drill.  Make sure you use the correct size drill bit, otherwise you will have a loose fit.  Once you've made your holes, you can use a screwdriver to fix the handles to the door.

If you're constructing a run of fitted bathroom cabinets, build each one individually.  Then line them up along the wall and use a spirit level to check they are in alignment, both vertically and horizontally.  Once you're happy with your units, you can do up the connections and secure them against the wall.  If you're fitting a sink vanity unit, prepare the water supply and drainage outlets and install the relevant fittings for the sink.  It often makes sense to fit faucets before installing a unit to the wall, as it will be easier to access the underneath of the sink before it's fitted.