I had been planning to trim a bed jacket with box-pleated trim for some time but wasn't sure how to do it. None of my antique sewing manuals were of help; the closest I got was the Harper's Bazar reprints in Frances Grimble's Reconstruction Fashions book. They illustrate complex pleatings but don't say how to achieve them!
Turns out there's help!
If you want to box pleat narrow trim by hand, it's simpler than it looks. All you need is pins and the fabric to be pleated. What you do is to make little accordion (knife) pleats of the same size, one to the right, then one to the left, and then one to the right, and then one to the left, and so on.
To see how it's done, have a look at Jennie La Fleur's YouTube tutorial at http://www.youtube.com/watch?v=DbJyLaHS6b4. Thank you, Jenny!
To make box pleats by machine, you can use a fork to manipulate the fabric to the left and right, just in front of the sewing machine needle. Heather McNaughton demonstrates this on her Truly Victorian pattern site. See her Help and Answers page at http://trulyvictorian.netfirms.com/FAQ.html and click on Fork Pleating. The video is in Quicktime, takes forever to load. Be patient. I had to wait nearly an hour to download it.