The trick is to start out by making the top edge that you’re pleating dead straight. Pull a thread in the fabric as a guide to cut along.
Measure the top edge and measure your waist: clearly, your task in pleating is to reduce the former measurement to become the latter.
When measuring your waist, plan for the skirts to fit tightly. When you lace the bodice over them, your waist will reduce in size a little.
Here’s how it works for my measurements:
Width of fabric: 330cm
My waist: 75cm
Notice that I’m going to need to reduce the fullness in that top edge to about a quarter of its present measurement, and then I’ll still have some left over. (You may find you need to reduce yours to a third or a half, depending on fabric width and waist size.)
We’ll worry about the bit left over later, but for now, keep in mind that we’re reducing the skirt width to a quarter of what it was.
Decide how wide you want the pleats to be. If you’re using metric measures I’d recommend making them 2cm wide. If you’re using imperial, make each pleat 1” wide when finished.
Just consider one pleat now. If my finished pleat is going to be 2cm wide, and we know that this has to be a quarter of the width of the fabric that was originally there, then we know that 8cm of the skirt's full width has to become 2cm.
Lay the skirt out along the table with the top edge running side to side in directly in front of you. Mark the centre front of the skirt at the top edge with a pin (I use pins with coloured heads - I use a red one to mark the centre front so I don't get confused). Measure out 8cm from it and mark with a pin. Measure again 2cm from the centre front and mark with another pin. By bringing the two pins together, you'll have made 8cm into 2cm. Can you see how this works? Diagram
Many period shapes had extra fullness at the back and sides, so you’ll be fine to make the pleats bigger towards the back.
If, by some fluke, your waist measurement is exactly a third or a quarter of the full fabric width, the pleats will be the same all the way round. Otherwise, you'll have a bit left over, meaning that your pleats need to become a little bigger towards the back.
In my case, for example, if my waist measurement of 75cm was exactly a quarter of the fabric we've got to work with, I'd have a 4x75 = 300cm wide skirt. But my skirt is 330cm wide - I'm going to have 30cm left over.
So what do I do with the excess? Our pleats near the back will just need to hide away a little more fabric. You can do this however you choose - I could decide in this case, for example, to hide away an extra centimetre in each of the last thirty pleats. Instead of measuring 8cm and 2cm, I'd be measuring 9cm and 2cm for those last few pleats.
However you choose to hide away the excess, just make sure you do the same on both sides - in this case, I'd add an extra centimetre to the last 15 pleats on each side.
(Alternatively, I could decide to add an extra 3cm to each of the last 10 pleats, for example, concentrating much more fullness at the back.)
So now all I need to know is where to begin making those bigger pleats. I can do this by working out how many pleats there'll be in total. Dividing my waist measurement by the width of my pleats, I get 75/2 = 37.5 pleats. I'll round up to 38, meaning that I'll have a total of 19 pleats on each side.
Don't fret too much about making the calculations perfect - you can always fudge your last couple of pleats a little to make the fit perfect.
So now I know that my first four pleats will need to measure 8cm and 2cm, and the rest will be 9cm and 2cm.
Mark further pleats as before, according to your calculations. Starting at the centre front, pleat each half of the skirt separately. I tend to make my pleats point towards the centre back (this does mean that you'll end up with what looks like a double width pleat at the centre front, where the two centre front pleats point different ways).
Pin a length of the skirt first, then pleat a few, checking that the finished row of pleats really are each 2cm (1") wide (they never come out right the first time, it's not you!)
To make the pleats hang straight, just make sure the top edge is always in line.
When you're about five pleats away from the centre back, measure the whole pleated skirt so far and compare with your waist measurement. Make your last five pleats make up for any discrepancies, ensuring that you end up with a skirt that matches your waist measurement.
Finally, baste and sew firmly over your pleats twice. Two rows of stitching will help to keep them straight.