You should know how to put stitches into hold position/ partial knitting and knitting buttonholes. The jacket is knitted in one piece with the help of waste yarn and nylon thread.

Of course it is possible to use a thicker yarn. I have added a hand drawing picture with measurements from A to H for help within this-

For the size of 3 months it is enough with 3 balls a 50 g.

These cardigans have been given away to my grannies, and their mother was satisfied! :-)