Find some large geodes. Extract them while digging for crystals with a trowel. Buy them at caving stores or other shops.
Dig a small hole in the ground that is large enough to accommodate the geode you want to crack. Place a layer of cloth in the hole to catch any crystal that might fall from the geode and to keep the geode from getting dirty.
Hold the geode in one hand and a chisel in another. Score the chosen rock around its entire circumference by dragging the chisel all around its crust.
Place the geode gently on the cloth in the hole. Make sure it is stable in the place by rocking it around a bit.
Strike a mallet firmly on a chisel whose flat edge is placed against the score mark. Place the chisel on another section of the mark by turning the geode. Strike the mallet again. Continue along the entire mark until the geode cracks open.
Have patience while striking each section of the mark because it will take some time before the geode cracks. This basically depends upon the thickness of the crust.
Open the geode gently. Slide the chisel between the two halves. Twist slowly to find the hidden crystals.