Lots of pins; lots of practice; lots of math. And lots of starch if you’re pleating cotton. Finally, lots of basting threads.
I find it’s easier to pleat on fabric with a repeating pattern because you can line them up like plaids or stripes (google ‘kilts pleating to stripe pleating vs pleating to sett’). Otherwise if you have plain fabric you’ll need to use a marking pen. Google, again, has lots of advice on how to mark for pleats.
Claire’s clothes have different kinds of pleats. The green tartan gown is knife pleated (which tend to be the easiest pleats), the wedding dress has cartridge pleats (which is a lot like smocking pleats–the pleats on those special-occasion little girl dresses). The waulking skirt has knife pleats but what makes it interesting is that the pleats are facing in different directions.
Just grab some fabric and start practicing!