The taper on the aprons is - I think fairly standard stuff - so you can probably find additional useful stuff like a diagram or a picture elsewhere.

I don't think you will be able to add in the taper very easily on an already done kilt - the 4 inches have got to come from somewhere.

If you were going to sew in the taper, you would measure your apron width, plus 2 inches and fold right side to right side at that point. Put in pins to hold the fold on a straight thread of the cloth - the straight grain, in sewing terms.

I then draw the sewing line, 2 inches from the fold at the waist, then gradually out to nothing at the hem. There might be some other way to do this other than a dead straight line, but a straight line works for me. You might need to consider how the buckles fit and dog leg the line to sew.

Sew along the line.

Turn the kilt right side out and press the edge carefully to get it flat. You have now 'lost' 4 inches of fabric at waist level so you really need to factor it in from the start, or in an already made kilt steal it from an under apron pleat if that is feasible.

There is a flare of 2 inches at the edge of the aprons, and the waist edges of the aprons are turned slightly uphill when worn, and the front of the kilt is very very slightly conical rather than swinging inwards from waist to knee.