Note: images are not drawn to the same scale.
To size the helm, measure your head at the temples, and add 1/4" for padding. Make the pieces half that long along the lower edge of the top pieces, and the upper edge of the bottom pieces (i.e. the long edge of each piece), but add 1" to each for the overlap. Add another inch to the front plates if you wear glasses, or have a really large nose. The vertical measurement for the top rear is roughly 1/3 of the horizontal -- more, if you have a tall head; less if you have a stubby head. Base the front verticals on the back measurements. The lower plates are roughly 1/3 longer vertically than the top ones.
Put the top plates together first, and lay them upside down on your metal
to make the top cap. Trace it out, and add 3/4" for the overlap.
Attach that, and dish it out if you like. Then attach the lower front, and finally the lower rear.