To complete a case with 100% skill you'll have to do the following things:
make sure you select the right tool the first try.
With the DNA minigame you have to search for the right result by placing the dna strands in the correct position. You can select every result without penalty but when actually placing the strands you have to get them all in one try. The easiest way is to count the colors. if they all match in terms of quantity you can begin placing them.
when you search for a chemical solution you get many choices of bars on the right of your screen. You will have to select some of them to copy the already existing image and press ''Compare''. Make sure that you do not select the wrong ones. Even without comparing, selecting the wrong bars will cost you some percentage.
When preparing a search if you have a partial fingerprint always select only 3 of the available 5 marks. Unlike the chemical you can't do anything wrong as long as you don't press ''Compare'' with the wrong results. You can freely drag the prints anywhere to check where they belong.
If you are not sure wich tool to use when collecting evidence or afraid you're gonna make a mistake in the lab, just make a save before you try. If your percentage goes down you can always load the save. Your skill percentage is not reduced if you make a mistake with the video, audio, medical, computer analize, both microscopic comparisons or the working table used to piece back together broken evidence.