Questioned documents in terms of deviant crimes are usually in the form of ransom notes, correspondence with authorities in order to taunt, humiliate, evoke fear, or to record the crimes for fantasy purposes.  When there is a suspected individual, a sample of their handwriting will be obtained to compare against the questioned document.  Attempting to determine a person's personality is a completely different process and will not be discussed here.

To the left is a excerpt from the ransom note left in the JonBenet Ramsey case.  Many handwriting analysts have studied this and compared it to the victim's parents, but have not come to any conclusions.

To make a comparison with handwriting, investigators can request a sample from a suspect spontaneously or in advance.  The obvious problem with requesting a sample in advance is that people have a chance to practice disguising their natural handwriting style.  Requesting a handwriting sample without notice does not allow the person to practice, but a person still will be able to attempt to disguise their handwriting.  Investigators can ask for previous samples of a person's handwriting on checks, letters, or work documents, however the drawback to using this method is investigators did not actually see the subject write the sample and may have a difficult time proving the sample is authentic.

When making a comparison, an examiner will look at the slant of the writing, where the person begins or ends a letter, the height and width of a letter, and the spacing between letters.  Other variables include whether there are certain words that are repeatedly used within known samples of a subject in comparison to the questioned document and the use of punctuation. 

The sample on the left is a copy of just one of the letters sent by the serial murderer that called himself Zodiac.  There are many samples that were sent to authorities over the years from the Zodiac and in at least two cases letters were received that proved to be hoaxes through handwriting analysis.

A very good exercise in practicing handwriting comparison is to use letters sent to the media or authorities by several serial murderers such as David Berkowitz (Son of Sam), Zodiac, and Jack the Ripper.  Another interesting exercise is taking a sample of one's own handwriting and attempting to reproduce it exactly and then trying to identify areas where there are deviations from the original.

Questioned documents can also include samples that are made from typewriters, computers, photocopiers, and may also include photographs that appear to be manipulated.

References

James, S.H, & Nordby, J.J. (Editors) (2005).  Forensic science: An introduction to scientific and investigative techniques (2nd ed.).  Boca Raton: Taylor & Francis.