The havelock helmet cover is basically a bit of fabric that fits over the whole helmet including any brim
The cover has an elasticated edge
Which you simple stretch over the helmet, so it takes a couple of seconds at the most to fit.
Some have a drawstring to the edge which does the same function as the elastic, just a bit more fiddly
This one unfortunately also included its own peak which did not compromise the fit but did look a little odd once fitted.
The Havelock also came with a selection of 3 different press-stud on pompoms so you can change the appearance a bit.