Seam allowance is the margin of material between the stitch line and the cut edge of a pattern piece.
The puppet standard is a quarter inch
— noticeably smaller than the five-eighths used in garment sewing. Puppet pieces are small and heavily curved, and a wide allowance bunches up inside a tight curve and pushes a visible lump through the skin.
Consistency beats the exact number
A quarter inch everywhere works. Three-eighths everywhere works. A quarter on some pieces and three-eighths on others produces a head that will not close, and finding out which piece is wrong means unpicking everything.
Add it after you trace, not before
Draft the pattern to the finished shape first, then add the allowance around the outside as a separate step. Building it in while drafting is how a piece ends up with the allowance counted twice, or not at all.
Clip curves
On a concave curve the allowance has to stretch to lie flat, so cut small notches into it, stopping just short of the stitching. On a convex curve it has to compress, so cut small wedges out. Skip this and the seam pulls and puckers no matter how well it was sewn.
Trim after sewing
where bulk is a problem, especially at points and corners. Trimming diagonally across a corner before turning it right side out is the difference between a sharp point and a lumpy one.
Mark it on the pattern
Write the allowance on every piece along with the napNap is the direction the raised fibres of a fabric lie — p... direction. Six months later you will not remember.
The hack: cut a small gauge from card at your chosen allowance and keep it at the machine. Eyeballing it is where inconsistency comes from.