They could've described this in a simpler manner. Describing the process is much harder than the process itself. Let me see if I can simplify it for you. First, put your four fingers together (excluding the thumb.) They don't say which of the set of knuckles they are referring to, but I interpret it this way... measure the widest part of the finger section of your hand across all the middle knuckles. (Each finger has three knuckles.) Ignore the palm part of the hand.
"Around and over the knuckles" refers to measuring across the middle knuckles of all four fingers all the way around the front and back of the hand. So, for example, if you start measuring at the middle finger of the back of the hand, you would wrap the measuring tape around to the other side of the hand and then continue until the tape meets again at the starting point.