On this page of the game guide to Borderlands 2 you will find a detailed description of the side mission titled Doctor's Orders. You can start this quest in the Sanctuary location during the completion of the main mission named Bright Lights, Flying City. You also have to complete the main goals of the storyline mission Wildlife Preservation.
Wildlife Exploitation Preserve
2) Slag Experiment Note 1
3) Gate/Super Badass Loader
4) Slag Experiment Note 2
5) Hyperion headquarters/Slag Experiment Note 3
6) Slag Experiment Note 4
Accept the mission  and use Fast Travel to reach Wildlife Exploitation Preserve. If you haven't activated Fast Travel there yet, head to The Highlands Hyperion Bridge and to the passage in the eastern part of the map. The first note can be found in the office  shown in the above screenshot, on a desk.
You will reach a locked gate . Keep fighting with all the appearing Robots and Stalkers (including a Badass) until a passage opens and a Super Badass Loader appears. Hide behind covers when it shoots and wait for the gun to overheat. Use the time between them to attack the Robot.
The next Slag Experiment Note can be found in the room marked as , inside a cardboard box from the above screenshot. Be ready for a fight with a Loot Midget after opening the box.
After reaching the next Hyperion base , you will have to kill a certain amount of Skags before the entrance opens. Head inside and you will see a Robot behind a desk. Climb onto it and press the button from the above screenshot. Open the locker behind the cover which was raised and inside it you will find Slag Experiment Note 3.
In the warehouse, head up the stairs to a room with cages . Use the lever from the screenshot to open them. In the last one you will find the fourth Slag Experiment Note. Now return to Tannis in Sanctuary  and collect your prize.