What kind of extract are you talking about?

There are a few extracts. Kif is one of them. That is the little crystals you see on the buds just collected into one. Commonly, it is preesed using extreme pressure and heat, forming something known as hash.

But you said liquid. Okay, liquid form you mean for cooking, or smoking?

You can extract the THC from cannabis using something that is high in fat. It is typical to use butter, oil, or milk.

The ohter liquid form is known as hashoil. One of the purest ways to smoke. That is made either using Isopropanol Alcohol, or Butane. You soak the cannabis matter in it, then what you are left with, is a greenish liquid. You can then either let the liquid evaporate on its own, or using heat to evaporate the liquid. After all the liquid has been evaporated, you are left with either a darkish, brown oil (using Isopropanol) or a redish oil (if you used Butane).

Keep in mind that both of these chemicals are VERY highly flammable, and should be used with extreme caution. Don't heat away the chemical indoors, and don't use an open flame to do so. Use an electric burner, and do not let the mixture boil. It will overheat, causing it to combust into a flame.

Be careful and good luck.