Summarize this article:

One round of bleeding the slave cylinder is complete when your friend lifts their foot off of the clutch pedal.  With the bleed valve closed and your friend’s foot off of the pedal, open the brake fluid reservoir on the master cylinder and add brake fluid to it to ensure it doesn’t run dry as you go.  Bleeding the slave cylinder with an empty reservoir will suck air into the system again. Check the reservoir regularly to ensure it does not run out of fluid. As your friend presses on the clutch pedal, look at the clear hose so you can see any debris or air bubbles that drain with the fluid.  Air bubbles may look like bubbles, or they may look like gaps in the fluid as it passes through the hose.  When there are no air bubbles for a few round of bleeding, you are finished.  Air bubbles in the system can lead to your clutch system only delivering intermittent pressure. Intermittent pressure in the clutch system can cause the clutch to fail to disengage. Do as many rounds of bleeding the clutch as necessary for the bubbles to stop coming out of the hose.  If bubbles continue to appear no matter how much you bleed the system, there may be a leak somewhere in the lines or at the master cylinder.  Once the bubbles stop coming you can seal the bleed screw and place the cap back on the reservoir. If there is a leak somewhere in the system, you will need to seek a professional to identify and repair it.

Summary:
Add more brake fluid between each round of bleeding. Look for air bubbles in the hose as it drains. Repeat the process as needed.