I just love these vintage potholder patterns. Next time I make this potholder I will probably up my hook size to a J hook. Mine was made with two sides - double sides using cotton yarn.
There is a video tutorial on you tube that helped me do up this tricky part of the potholder design. It’s in Russian, but if you have basic knowledge of crochet stitches you will be able to follow this. If you only need help with the last part of the joining - fast forward the video to about the 30:13 mark. This was such a help for me. https://www.youtube.com/watch?v=Arb_T0Uka-M