If it's just dried lumps, break or chop it into the smallest pieces you can, add a little water and mix. Be careful not to over water or you'll get slip. Put it in cling film or Saran Wrap, let sit for a few hours or overnight, knead it through the plastic and add more water. Keep kneading, adding water and/or drying until you get the desired consistency.

There are a number of ways to speed up the rate at which it soaks up water. The smaller the pieces, the quicker it will turn back to clay. A coffee grinder kept specifically for the purpose is great for getting the lumps into a powder. You can also use a mortar and pestle. Chopping the lump using a tissue blade or craft knife also helps to get small particles which absorb the water more quickly.