This is on the 15th Level, "Skurky's Law", for completing the first (of three) sections. Easiest path seemed to me (played on "hard"):
- From the courtroom, turn around and take the next right (not upstairs), leave the building through the door.
- Turn right and enter the library section, sneak downstairs, wait for the officer to turn his back to you, subdue him, take his disguise and hide the body.
- Exit the room through the window (officer should still speak to the intruder), and just walk straight.
- When you reach the yard with two officers talking, just sneak to the right behind the parked car and pull yourself up the ledge. No need to distract anyone. Enter the window.
- Wait for the trial behind the doors to reach the part where the television gets switched on. Switch it off again by using the computer nearby, and head left into the judge's room.
- Wait for the judge to enter the room. You are safe there, he won't be alerted. Wait for him to go to the back part of his room, subdue him, take his disguise, the keycard, and the hammer (don't forget the hammer!). Hide his body in the wardrobe (don't get spotted by the lady downstairs).
- Enter the courtroom, wait for the defendant to reach the room, and pass sentence. (One of my favorite situations in the whole game.) This will also unlock a challenge.
- Go right and then left towards the locked door, unlock it and enter the jail section.
First guide, be gentle, please.
Edit: A somewhat easier solution would be to finish the first two steps, then go back to the courtroom the regular way, enter the evidence room and climb through the vent. But that's so much less fun... ;)