This knot is also called “sheet knot” or “net knot”. But it is universally known as flag knot. And the reason is clear: it works well to join two ropes of different thickness and it lasts if you always use it in traction. So, it’s good to fix the flag on the hand line. It’s not good, on the contrary, to join two mooring ropes (in this case, in fact, two bowline knots are better) because the continuous sequence of tractions and releases would risk to untie it.
How can we do it? At first, we have to take the first rope (the largest one if they hae different sizes) and do an eyelet.
We put the running part of the second rope into the eyelet and above the running one of the first rope.
Then, we have to pass the running part of the second rope under the first rope and put it into the eyelet again. If, at this point, we leave the current part parallel to the sleeping part, we will have tie a flat knot.
For a flag knot, we have to pass the running part under the sleeping one and then tie.
The knot is done!