tfurrows, re: measuring for outlets, light fixtures, etc.

To begin with, here are some general principles: think of 
everything as a grid. Rely on a square to find the positions 
in the grid.

Here's a little ASCII art depicting a traditional L-shaped 
square to convey the general idea of what follows. But it 
really helps to have a full size (48") drywall T-square for 
doing this.

-------------------
|                 |
|                 | |
|                 | |
|                 | |
|                 | | <-- square
|                 | |
|                 | |
|                 | -----------
|                 |        --
|                 |       |  | <-- fixture
|                 |        --
|                 |
|                 |
|                 | <-- neighbouring panel
-------------------


Install panels until you get close to the proposed cut-out.

Let's say that you're installing the panels vertically and 
you've installed panels from left to right along a wall. 

Put one side of your square against the right edge of the 
neighbouring panel and line the other up with the bottom of 
the fixture that you need to cut a hole for. Then:

1. Using a pencil, mark the point on the neighbouring panel 
parallel to the bottom of the fixture.

2. Using the measurements on the square, write down the 
distance from the neighbouring panel to the close and far 
edges of the fixture.

3. Move the square up to the top of the fixture. Mark the 
point on the neighbouring panel parallel to the top of the 
fixture.

4. Measure the distance from the bottom of the neighbouring 
panel to your two marks. 

Now, when you measure for the hole on the new panel, the 
distances you measured up the neighbouring panel are the 
measurements (from the bottom of your panel) for the bottom 
and top of your hole. The distances that you measured over 
from the neighbouring panel in step two are measurements 
from the left side of the panel to the the sides of your 
hole. If you just draw out the lines on the new panel (using 
the square), they will intersect at all the corners.

If your hole is round, find an appropriately sized container 
lid and center it inside the lines. Then trace around it. If 
the hole is hexagonal, just wing it by drawing in the cuts 
across the corners (or grab another electrical box and trace 
around it).

You can also make all of the measurements out of a corner, 
or from the ceiling downwards, etc.