Put on goggles and protective gloves. While chiseling does not always result in rock pieces flying, it is possible to have small pieces of rock chip away and fly upward while you're working. The goggles protect the eyes from potential flying rocks. Fingerless gloves also protect, but leave the fingertips free for optimal control.
Determine the design for your sandstone slab. Designs can range from simple tiles or rocks carved down to manageable size to artistic statuary. If thinking of an artistic design for the sandstone slab, take care to look over any flaws or cracks in the stone, and any other problems that might arise. Since sandstone is a relatively soft rock, minor cracks and flaws can create big problems for a design. Examine the rock to ensure the design will not have any problems.
Place the chisel against the sandstone slab so that the sharp and flat side of the chisel is facing downward. Close a hand around the chisel about halfway down and keep the thumb down with the rest of the fingers to prevent hitting the thumb.
Tap the chisel with the mallet. Avoid hard blows with the mallet, which will cause the chisel to penetrate the sandstone too deeply and result in cracks, flaws or ruined sandstone. According to The Sculpture Studio, the chisel should not penetrate stone too deeply, or it can ruin the design or result in more work to remove flaws and problems.
Chisel parallel lines that are close together. Remove small areas of sandstone by changing the direction of the chisel and cutting the same area in a cross-hatch cut.
Work in small areas, from the outside of the sandstone toward the inside. It is especially important with sandstone to work in small areas and chisel away gradually to prevent the stone from crumbling. Take extra care when working around previous cracks or flaws in the stone.