I did this on my 1997 Carrera at 37,000 miles.  There was no sign of leakage from the old cylinder, but my car was suffering from the well-known notchy and noisy clutch pedal problems.  Apparently the 993 slave cylinder does not have a long service life.

The slave cylinder is mounted on top of the transmission, so access is awkward and most of the work has to be done by touch with one hand. This DIY would be much easier if you could see what you were doing and could use both hands.  Unfortunately it has to be done blindly and you can only use one hand.   The actual operations are quite straightforward after you have worked out how to access the area.  It is well worth the effort to have a clutch pedal that operates smoothly and quietly.

Symptoms

The clutch pedal feels notchy, and the slave cylinder squeaks when the clutch pedal is depressed and released.   The problems are not solved by lubricating the clutch pedal or by bleeding the slave cylinder.

Tools, parts and materials

Vise grip
Phillips screwdriver
13 mm Wrench
17 mm Wrench
950 116 237 12 Clutch slave cylinder
(Optional: flexible hydraulic hose to clutch slave cylinder)
Synthetic grease

Procedure

A.  Preparation

1. Use a vise grip to clamp off the clutch master cylinder hydraulic line (blue fabric covered line) where it exits the brake fluid reservoir.
2. Jack up the back of the car, (How to jack up the car) support on jack stands and remove the left rear wheel.
3. Undo the Dzus fasteners with a Phillips screwdriver and remove the engine and transmission trays. 
4. Find a position under the left rear of the car where you reach up and find the slave cylinder.  Unless you have the luxury of a lift, the easiest way seems to be to lie on your back with your feet towards the back of the car and your shoulders just forward of the suspension.
5. To improve access slightly, undo the two 13 mm bolts securing the bracket that holds the flexible breather hose onto the top of the transmission housing just above the drive shaft.  Leave the bracket clipped to the hose, and slide the bracket towards the front of the car so it is out of your way.  This hose and bracket have nothing to do with the slave cylinder.  You are just moving them to make it easier to reach the slave cylinder.

B.  Removal

1. Loosen but do not attempt to remove the 17 mm coupling that attaches the flexible hydraulic line to the body of the slave cylinder.
2. Refer to your new slave cylinder to understand where to feel for the two 13 mm nuts that secure the slave cylinder to the mounting studs on the clutch housing, and then remove both nuts.  Note that the return spring inside the slave cylinder will push the slave cylinder away from its mounting points as you undo the nuts.
3. Slide the slave cylinder forwards to clear the mounting studs, and drop it down so it is hanging on the end of the flexible hydraulic hose.  Inspect the condition of the flexible hydraulic hose, and take this opportunity to replace it if needed (not covered by this DIY.)
4. Unscrew the flexible hydraulic hose from the slave cylinder.  Have a rag available to catch any drips of fluid.
5. You can now reach up and feel the clutch release lever through the opening where the clutch slave cylinder was mounted.  Locate the recess in the clutch release lever where the slave cylinder pushrod fits so you will understand how the components are supposed to align when you install the new slave cylinder.

C.  Replacement

1. Screw the flexible hydraulic hose coupling into the new slave cylinder and tighten as much as possible by hand.
2. Remove the slave cylinder bleed screw cap and loosen, then just hand-tighten, the bleed screw to making bleeding easier later. 
3. Place a small dab of grease on the rounded tip of the slave cylinder pushrod.
4. Lift the slave cylinder into position.  Make sure the flexible hydraulic hose is not twisted.
5. Slide the slave cylinder towards the back of the car so that the pushrod engages the recess in the clutch release lever and both mounting studs align with the holes in the mounting plate of the slave cylinder.
6. Press the slave cylinder towards the back of the car against the resistance of the return spring.  Get the cylinder to “catch” on the threads of one of the mounting studs to hold it in position against the spring resistance so you will have some exposed thread on which to start the first nut.  Tighten the first nut to draw the slave cylinder against the mounting to expose some thread on the other stud so you can start the second nut.  Tighten both 13 mm nuts securely.
7. Tighten the 17 mm coupling that attaches the flexible hydraulic line to the body of the slave cylinder.
8. Remove the vise grip from the clutch master cylinder and bleed the clutch. (How to bleed the clutch)
9. Test for smooth pedal action.  The pedal action should be smooth, silent and lighter than before.  If the clutch pedal goes straight to the floor and does not return, the system needs to be bled again.
10. When you are satisfied that the clutch is working correctly, replace the bleed screw cap and re-attach the breather hose bracket.
11. Replace the transmission tray and the engine tray (optional).   Replace the left rear wheel, lower the car, torque the left rear wheel nuts and go for a drive to enjoy the fruits of your labor.