The easiest way would be to use a fletching jig like Bitzenburger, for example.
Helical fletching is IMO surperior to straight fletching as far as stabilisation of the arrow is concerned. And THAT's what the fletching is all about, isn't it!? You may be able to get the same effect from 3.5" helical as you now get from 5.25" straight ...
On long shots (clout etc.) you'll notice a good reduction in flight distance, due to higher air resitance. Nothing to worry about at normal shooting ranges.
For years I have done my fletching without a jig! It can be done - and quite consistant with some practice! Pin the quill done to the shaft with needles (those with glass beat heads). It helps to mark the start and ending spots on the shaft - and using a vise to hold the latter.