An automatic cord knitting machine is used for these DIY knotted coasters, but links are provided for making handmade knitting spools and how to knit an icord.
What is spool or French knitting?
From Wikipedia: Spool knitting, corking, French knitting or tomboy knitting is a form of knitting that uses a spool and a number of nails to produce a narrow tube of fabric, similar to i-cord. Spool knitting is a traditional way to teach children the basic principles of knitting.
Lots of the DIY Harry Potter inspired Knitting Patterns are free, some are pay patterns. Here are free 2 of my favorites from this roundup: