I trained at Bart's, London 1980 - 83. The pleated cap was worn by students and 1st yr qualifiers. Then you went on to the '5th yr cap'!
The pleated cap was formed by taking a well starched rectangle of white linen, lay it on a flat surface with the long side closest to you, take a strip of cardboard, about the circumference of your head and about 5cms deep and match its edge to the edge of the cloth (in the middle of the length), then fold that side up and over the cardboard twice to form a smooth, supported, cloth-covered strip.
For the next bit you need something round and hard - the bottom of a metal saucepan or as many of us used, a small, round shortbread tin.
You form the strip containing the cardboard around the tin and secure underneath with a small, 1cm, safety pin. This leaves the rest of the material as a single layer sticking up. Working from one side to the other, you smooth the material down into 5 or 6 triangular pleats, then pin together the top of the strip containing the cardboard over the pleats trapping them between the two safety pins. You are then left with a 'tail' sticking out from between the safety pins which you fold up and smooth flat over the top of the cap.
Way easier to do than explain, we taught each other!
Caps were abandoned by the end of the 80s, deemed an infection risk and outdated frippery. :chuckle