This is a collection of PDF pattern files that give you the base for the pictured helm. 
 
When downloading the file, print it off on a standard 8.5" X 11" sheet of paper and it will be sized for a 24" circumference. For larger or smaller heads, add or subtract an eighth of an inch to the pattern on all sides to change the size by one inch.

When measuring your head, remember to leave room for padding.