Skill is measured in the amount of attempts it takes you to match DNA, fingerprints, materials, and chemicals correctly. To get 100% you will need to match the evidence correctly without making any mistakes.
An easy way to get 100% skill is to save before using any lab equipment that effect the skill statistic. Then, when you know the correct match, reload your previous save and match it up and then save your game again.
These achievements do stack so getting 100% skill will also get Test Tube Washer and Research Grant as well. To check your skill percentage click on the PDA, and then onto case file, and then go to statistics.
Use this guide to get this achievement easily on case 2: Guide.