Leaching is a process where we remove excess oils and plasticizers from newer (aka "Fresh") polymer clay. The trick is literally plain old uncoated copy paper.
Take half of your soft package and roll it to less than 1/8" thick (2mm). Place the sheeted clay between two pieces of copy paper and put some weight that will cover the entire portion. Some folks literally sit on it! No, we are not joking! :O
The next step is to wait. How long? Well, this depends on the type of clay and the manufacturer. Some brands need to set overnight. Some, like Cernit Pearl, need to sit only minutes. Others, like Cernit Number One are formulated differently and require different times. The same goes for Cernit Translucents as they can be quite soft when first made. The point is, how would you know the difference? The best approach to determining the correct treatment is to test it.
Start with 3 minutes. After that time period has passed, roll the clay back up and mix thoroughly. The portions touching the paper will be very firm and the inside will be less firm. Mixing and kneading will restore the balance to the entire piece. Is the final product firm enough? If not, simply repeat for another 3 minutes. Continue until you have the half block firm enough for your needs. Now, if you went too far, simply mix the other half of the untreated package and work the clay again. This is really all by feel and you are the best judge of what you need.