A scutch comb is a double-headed toothed metal tool -- basically, a removable chisel head. Instead of buying a scutched chisel, you can invest in a scutch chisel or hammer holder; when the toothed head wears down, simply replace the scutch comb instead of purchasing a whole new tool. Scutch combs are relatively simple to use, but they must be used correctly; sharp tools are always potentially dangerous.
Insert one end of the double-headed scutch comb into the bit holder of the scutch holder or scutch hammer.
Push in the scutch bit until the joint between the two heads rests on the lip of the bit holder. A work-gloved hand may be enough to push it in, or you may need to tap it in gently with the flat side of your hammer. Make sure that the scutch bit is centered in the bit holder to avoid slippage.
Use the scutch holder as a chisel (or the scutch hammer as a hammer) to roughly carve and loosely shape the stone.
Knock the scutch comb out of the bit holder sideways with a hammer to remove it.
Flip the scutch comb over and insert the dull side into the bit holder following the instructions in Step 2.