Slip is just another name for liquid clay. You can buy it ready made or mix it from powdered ingredients, but the simplest way to make it is to use the dried clay leftovers from trimming pots.

The process really couldn't be much simpler:

Put your scraps in a container.

Add water.

Leave for a little while for the clay to get saturated, then pass through a sieve. This one is 60s, which is probably finer than necessary.

At this point it will be silky smooth, but might have more water than you want in it. In which case you just leave it overnight and remove the water that comes to the top of the slip.

This can now be coloured using stains or oxides and used for a range of processes (such as Drippy Slippy or Nautilus pieces).

The stains should come with instructions from the manufacturer on how much to use and what other factors to consider (for example, pink and purple stains won't work in my clay as there is probably too much zinc in it for the chrome-tin stains to produce the correct colour).

The colours I use oxides for are blue slip (0.25-0.5% Cobalt Oxide) and black slip (1% Red Iron Oxide, 1% Manganese Dioxide, 0.25-1% Chrome, 0.25-1% Cobalt Oxide).