To get 100% thoroughness, you will need to visit every navigable area in a certain case You will also need to check that you have got every bit of evidence and that it has been processed fully. To check this click on the PDA and then click on the evidence icon. If you have processed the evidence fully it will have a green tick on it. You will start off at 0% thoroughness and work your way up to 100%, when you have 100% thoroughness you have found every available location and processed all of the evidence. Some evidence cannot be processed and will remain unticked. Don't worry about it though, as it won't contribute to your thoroughness percentage in anyway. Look around the CSI lab to get at least 8% thoroughness, even going straight down the corridor will add thoroughness to your percentage.
These achievements do stack so getting 100% thoroughness will get you Thorough Enough, Darn it and Close Enough Counts After All as well. To check your thoroughness percentage click on the PDA, and then go on case file, and then go on statistics. Use this guide to get this achievement easily on case 2: Guide.