This is a story related achievement which will unlock with natural story progression.
After you have finished at the hospital you will find yourself back at the apartment and playing as Holmes. You will need to make your way to the Wasps Nest Pub and collect the tweezers from the bar. Just to the right of the bar you will find a gap in the floorboards. You will need to complete a sliding pieces puzzles to collect an amulet.
There are 28 steps to this combination so follow this video carefully. There is no wrong combination but making a mistake may draw out the process.