In case it helps I'll let you know how I come up with the patterns. For a full-faced steampunk mask you can start with a doll's head, and add clay to it sculpted into the shape of the mask (like the gas mask for instance). Then cover it with masking tape, mark where the seams go, and cut the tape apart along the lines to get the individual patterns.
Tape them down onto cardboard and trace around them. Then you need to enlarge them to full size. I scan them and work on them in Corel Draw, smoothing the lines out, adding seam allowances where needed, adding holes for rivets and buckles, etc.
Most all of my work is handstitched. I space out my stitching slots as needed so that abutting pieces match. I stitch the pieces together while damp (veg tan) so that the leather can stretch or compress as needed to give smooth lines.
Using this technique you can get a pattern for just about anything. For my firemaster's helmet I filled a hardhat with plaster and then shaped it using a Surform to get the crown shape, taping it up the same way to get the pattern. Hope this helps.