How to make slip
Most of the time you can just score your pots (make hash tag etchings with your tools) then apply a little water and you should be fine, but for some shapes like mugs, butter dishes, big vases its best to make proper slip!
Making slip:
You can either dry a flat thin piece of clay until its bone dry then break it up and mix in a little bit of water OR just use wet clay. I also use little bits of tissue and vinegar and blend it all together to make a super slip that bonds super effectively (mainly for pieces I want to fire in a kiln and make them ceramic), but you can just use the clay and water for air-dry decorate pieces.