See "Master Technician" for more info.
Basically to get 100% skill score in a case you need to always select the correct fingerprint, click the identifying qualities correctly every time, and match it to the correct area. You also need to use the correct tools every time and choose the correct chemical matches.
If you misclick one or two items you will more than likely still get the 80% skill level required for this achievement.
The easiest way to do this would be to use a game walkthrough such as
http://faqs.ign.com/articles/105/1057755p1.html