Figuring out the pattern I wanted to use for the shirt was one of the biggest challenges I had. There are two ways to do the patterning: the pleats make the design or the threads do. They are essentially reverses of each other.
This handout from my apron class shows an example of using the threads to form the pattern. http://www.pleatworkembroidery.com/articles/apronintwoways
Here http://www.pleatworkembroidery.com/recreating I have a example of an extant shirt fragment and the recreated version.
You have to make sure that you do not use a pattern that carries the threads over too many pleats or the threads will catch on things and sag.
The other way to look at the pattern is to use the pleats to form the design. http://www.pleatworkembroidery.com/articles/15th%20Century%20Ladies%20Hemd
This is much more stable because the design threads are run through the majority of the pleats and only small stitches pass over the the pleats. I opted to go this route because after looking at shirts and necklines of similar design in the period artwork I decided that it would be closer to what I was seeing in these pictures.
The pattern is charted from an extant Modelbuch by Micholas Bassee. While the book dates from 1568, these patterns were widely used throughout German and Italian embroidery.
The dots are where the thread goes over the pleats, the white spaces are where the thread goes through the pleat:
The first row is the hardest because that is laying out your pattern. After that, you can use the stitches in the previous row as a pattern guide. Hint: When doing the stitching, it is easier to pull the needle through multiple pleats as shown in the image above. It keeps the stitches lined up better and the rows even.
This is the pattern after 10 rows. The pleats are pulled in very tight and the pattern is beginning to emerge.