Slip is a watered down clay. It is used in pottery as a "glue" to hold clay pieces together. It also lessens the chance of cracking when it is used instead of water when joining pieces of leather hard clay (clay that is partly dried and firm).
What you need to make it:
(1) Dry clay scraps
(3) Resealable lidded container
Put dry clay scraps in container. (figure 1) Add enough water to cover the clay. (figure 2) In about 10-30 minutes the clay will disintegrate. The smaller the clay pieces are, the faster this will happen. (figure 3) Stir well. The mixture will be as thick as soft pudding. (figure 4)
You've got slip!
Occasionally you will need to add water to the slip as it will dry out over time. Clay slip improves with age, getting smoother and creamier, and sometimes it will get downright smelly (like a swamp). This is caused by mold growth. Throw it away, if you want, but usually the slip that gets to this stage has a very fine texture to it. To kill the mold, add a few drops of chlorine bleach to the mix and stir well. That will eliminate the odor.