Article by Paul Wandless from ceramicartsdaily.org/ it goes into much more detail if you want to use slip for decoration etc. but a basic slip preparation for joining is below:
How to Transform Your Clay Body into a Casting Slip
Preparing the Slip
The process of slaking clay to make it into slip is a simple one. Step one is to have a 5-gallon bucket filled 60-75% with your bone-dry clay then add water until it’s a few inches below the rim (figure 1). Let it slake for 24 hours then mix it by hand a little with a stick (figure 2). Once you’ve mixed the slip for about 5 minutes, use a drill with a paint mixer attachment to mix it into a smooth slurry, which should be the consistency of thick cream. Use the stirring stick to check the consistency and find any lumps of clay that didn’t slake down (figure 3). Keep mixing until the lumps are gone.
…At this point, I normally fill a quart container with the slip to use as a joining or brushing slip.