Maybe I should use a jig.....but I've been doing my fletching by "eye" for a long time. Besides, with NA style arrows...I don't need to be too neat and I don't have to glue down the quills if I don't want to.
First I tie on the rear of the cock feather with a couple wraps of sinew or thread. If I'm using thread, I placed a dab of wood glue on the shaft and a bit of glue on the thread to hold it down. Then I add the other two feathers with a few more wraps. Then I adjust the spacing of feathers by eye. I leave enough quill on the back (and front) of the feathers so I can get a good grip on them when making the adjustments.
After the rear of the feathers has been wrapped down with more thread/sinew, I tie down the fronts with a few wraps, adjust, and pull tight. Then I finish wrapping down the front quills - usually about 1" of wrapping.
Oh yeah, I dip the feathers in water (and shake off the excess) just before I tie them. The water expands the feathers, makes it easier to push the vanes out of the way, and makes the quill less brittle. Feathers shrink a little as they dry....pulling them even tighter between the wraps.