The head of a parejo is closed with a cap — a small piece of wrapper leaf applied over the end. The shoulder is the point where the cigar's straight side begins to curve into that cap. Cut at or before the shoulder and the wrapper stays anchored; cut beyond it and the wrapper has nothing holding it, and it unravels as the cigar burns down.
The three cuts
A guillotine takes a straight slice off the head and is the most common; double-bladed versions cut from both sides at once and are less likely to tear. A punch removes a circular plug and leaves the cap otherwise intact, which suits a broad round head and does not suit a tapered one. A V-cut takes a wedge, giving a wider opening for the same depth.
How much to take
Less than expected. A cut can always be made deeper; it cannot be undone. Starting shallow and extending it if the draw is tight is the whole technique.
Figurados
A tapered head has no single right depth — the further down it is cut, the wider the opening becomes, so the cut is itself an adjustment of the draw.