Whether you're handbuilding, sculpting, or throwing on the wheel, for best success your clay must be soft, pliable, and void of air bubbles/pockets. Fresh pugs/bags of clay contain the proper amount of moisture for the particular clay mixture... approximately 30% by weight. The clay was mixed at the factory to a uniform consistency and formed with a vacuum extruder which removed any air pockets prior to being bagged. It is ready for use straight from the bag for handbuilding and sculpting, but these clays typically require wedging to improve pliability before throwing. Clay which has been in the bag for an extended time tends to lose moisture and often becomes too dry to use effectively. Collections of clay scraps/trimmings (saved from previous projects) may be either too dry or too wet, usually are not of a uniform consistency, and typically contains air pockets which can explode when fired. How does one get their clay to the proper state before beginning a project? If you're using clay from a fresh bag you're pretty much good to go as-is. If you're throwing, you likely will want to wedge it a little to improve pliability. Otherwise...
1. If your clay is too wet... spread it out on the plaster side of the wedging table to let the plaster draw some of the moisture from the clay. Check every 15-20 minutes as you don't want it to dry too much. If the clay was sloppy wet, leave it sit on the plaster until it will lift off without sticking much. Do NOT scrape it off the plaster as you'll risk getting bits of plaster in your clay. If you don't have access to the plaster tables you can spread the clay out on a concrete garage floor or simply let it air dry until you get the appropriate moisture content. Regardless of which method you used to remove the moisture, you'll need to wedge the clay to get good consistency throughout.
2. If your clay is a too dry... wrap it in a moist towel and then in plastic and let it sit for several days... the clay will absorb the moisture from the towel. It needs to sit long enough for the moisture to migrate uniformly throughout the clay... then wedge it to get good consistency.
3. If your clay is (almost) totally dry... put it into a plastic bag. Weigh the bag and calculate how much water you'll need for 30%... more or less depending on how dry the clay is. A cup is approx 1/2-pound. Put the bag of clay in a bucket (in case the bag leaks), pour in the amount of water you need, and close/tie the top of the bag shut. Let this sit for several days to allow the clay to absorb the water. You can't hurry this process... you must allow time for the moisture to migrate throughout the clay and equalize in consistency. The longer the better.
Once your clay is of the correct moisture content it's time to wedge. Wedging makes the clay more pliable, ensures a uniform consistency, and removes air pockets. Wedging takes some practice to master the technique, but wedging shouldn't be hard to do... doesn't require a lot of strength, etc. If you can't wedge then likely your clay is too dry or your technique needs adjustment. Remember how soft clay is from a new bag... that's the consistency you want to wedge. Watch this video by Bill vanGuilder and then practice various clay wedging techniques.