Hold the slave with the push rod end facing the floor. Ensure that the slave is the lowest part of the system. With the cap off of the master, depress and release the slave cylinder. DO this until no more air bubbles up out of the master.
DO NOT do this too quick, and DO NOT allow the slave plunger to fully cycle. This will ensure the seals do not blow.
One complete, install the slave to the bellhousing, put the cap on the master. Go.
Hope this helps.
Jim