Box pleats are a different touch to the standard gathering and pleats. I've used them for curtains and skirts, but I'm sure there are a thousand and one other uses for them. The three sets of London Blinds I've made use a box pleat to create the decorative pull-ups, and I've also made simple box pleat curtains to cover small windows in doors:
Step 1: Measure and cut fabric. For my curtains, the windows were 40cm x 138cm (16"x 55"). I added 5cm (2") to each dimension for hems/seam allowance and then 30cm (12") for my box pleat (which makes a 15cm (6") box pleat when folded together).
Step 2: Hem your project as necessary (this was required for my curtains but is not required for a box pleat for a skirt, for example).
Step 3: Mark out the centre-line and edges of your box pleat on the wrong side of your fabric. I sewed together 10cm (4") for my box pleats - once again, this will vary depending on your project.