The ideal object for this method of measuring is an object with a complex and otherwise difficult to measure shape but with at least one flat side so that it's easy to lay flat on a piece of paper. If the object doesn't have a flat side, being able to make one side flat will do the trick.

For example, this piece of plastic attaches to the back of a drawer to guide it along a rail so that it moves smoothly and straight:

The back of this piece is almost flat, except for 2 nubs sticking out, which would be easy enough to remove. Simply note the location of parts that are removed before removing them, since they'll need to be modeled back in.

To begin, use a sharp blade and cut the nubs off the mostly flat surface:

Then, to be sure that the surface is perfectly flat, use some fine-grit sandpaper on a flat surface to sand down the flat side:

Next, take some grid paper, centimeter if available, and lay the object on the paper. Line it up with the grids as best as possible, and trace the object. Keep in mind that traces are generally a little bigger than the actual objects.

As an alternative, an ink pad can be used to make an even more accurate shape transfer. Just use the flat side of the object like a stamp. Again, try to line the part up with the grid as much as possible when making the impression in order to make the later modeling process easier. Stamping on a soft surface such as a towel can help get better coverage in the stamping process:

Finally, scan the stamped part in, or if a scanner isn't available, use a digital camera—whatever is possible to get the traced-image-on-graph-paper reference into the computer. Try to keep the grid lined up and even in the image, or some editing of the image will be necessary. It is not possible to edit the image in the modeling software beyond some basic transformations, so keep it as straight horizontally and vertically as you can and avoid skewing and warping of the image as much as possible: