This article shows great examples and goes over the process, which is very simple: pin the item into the desired shape, then use a hair dryer on the high setting, being careful not to burn the yarn.
Beware, this will change the drape of the fabric and the look/feel of your garment.
Ooh I do a similar thing (very carefully) with our iron, but never even thought to try the hairdryer. I expect it takes a bit longer but is probably a lot easier, definitely going to try this out sometime.
I’ve also done it with steam from an iron, and the hair dryer seems a bit easier!