The most important measurement for the hennin is the circumference of your head, from the highest point of the forehead to the nape of your neck. Using this measurement, trace out a pattern for a headband/fillet. This can either be rectangular in shape or with a slight curve for a better fit. It needs to be tightly fitted as it is the main anchor point, so designing it not to quite fasten when tied together at the back a small gap should allow for any stretch. Making it up in velvet with an interlining of canvas creates a sturdy support as well as a rough texture for other fabrics to grip onto.
The black loop, which can be represented in paintings in various ways, can be created using millinery wire wrapped in silk ribbon, though it is difficult to ascertain exactly how it is constructed so other interpretations are worth considering! It can be sewn between the layers of the fillet fabric and sewn tight, and should be placed as high on the forehead as possible.
The measurement of the hennin's base will need to allow for the headband so it is worth measuring again. Using this measurement, a curved edge can be sketched out for the pattern. The sides and top of the hennincan then be drawn in after considering how long/tapering the hennin needs to be depending whether it's full length (approx. 50cm) or truncated (approx. 20-25cm). It is advised to make a mock up in cardboard (e.g. cereal box) beforehand.
The pattern is traced onto a sturdy buckram or equivilent and cut without seam allowance and onto a lining and out fabric with seam allowances. The seam edges of the hennin need to be finished before assembly in order to whip together the edges with all the raw fabric concealed. The outer fabric needs to be a bit longer than the hennin so that the fabric can be sewn to cover the hole in the top when later constructed. The seam can then be worked up the length of the hennin and the base optionally bound in velvet to grip to the headband if the hennin is particularly long or heavy.
The next thing to make is the veil. This can be made from a hemmed piece of silk organza or a very fine linen measuring from 1-2 metres (depending on how flamboyant one wishes to be) and the width would have to be measured accordingly depending on the length of the hennin.
The last piece to make is optional, but does appear in most paintings showing hennins: this is the frontlet, the long strip of velvet placed over where the hennin sits on the fillet. this is just a tube of velvet about 10cm wide and about 80-100cm long.