Binding fishing nets is an ancient craft that have been done for thousands of years. I will teach you the way that my grandfather taught me when I was a kid. He had been doing this as a kid for extra income to the family and for their own use.
Basically the technique is the same for big rectangular fishing nets and for small net bags or landing nets, the difference is just size and how you finish the net.
Step 1: Tools
You basically just need two tools: The shuttle (or binding needle) and the gauge.
The shuttle holds a length of yarn and is pointed so you can thread it between tensed pieces of yarn. The gauge decides the size of the holes in the net. Each square hole in the net will have four sides that each are the width of the gauge.
Start by winding up some yarn on the shuttle. For the first round of loops you should not have too much yarn on the shuttle if you are making a net with small holes, because that first round will as you shall see have half size loops, and then an over filled shuttle wont pass through.
Step 2: Starting
You start by making a loop of thread that you can hang somewhere, at a door handle, over a knob on a chair or a hook of some sort. Whatever that can be a steady hold. I made that loop of green thread in the pictures for clarity. Use a piece of thread at least 75 cm long so it doesn't feel tight.
Tie the end of the binding yarn to that loop as in the first image.
The first loop is then started by putting the gauge under the yarn and making a loop behind it like in the second image. The needle is threaded down through the green loop and out of the hanging yarn loop towards the right.
The loop is then tightened by pulling the shuttle towards you as in the third image.
Holding the yarn to the shuttle with your thumb, you "throw" a loop over your thump and the green loop, and then you thread the shuttle up through the green loop, under the yarn loop. Look closely at the fourth and fith image to see how this is done and where the yarn is.
Tighten the now full knot by pulling the shuttle towards you. Do not release the pressure of your thumb on the yarn, it should be kept all the time until the yarn escapes by itself when you pull the knot tight. See image 6.
Now you made the first loop of the first row of loops.
Step 3: Complete the First Row
Now make the second loop like the first one by making it towards you, then under the gauge, threading the shuttle down through the green loop and out towards right within the yarn loop, and tighten on top of the gauge. That's the first half of the knot.
Then hold it there with you thumb and throw a loop over the thumb and the green loop, and thread the shuttle upwards through the green loop, inside the thrown yarn loop. And tighten towards you.
That's the second loop of the row complete.
Now repeat this step until you have as many loops you need. The number depends on what kind of net you want to do.
If it is a landing net that is supposed to go on a circular frame, the net will be a cone where the starting loop will be in the middle and the width of the net will be decided by the circumference of the frame.
If it is supposed to be a rectangular net, it is usually the number of loops that will decide the depth of the net, and you then keep on adding rows to get the length.
In my example I just do 6 loops for illustration, which won't be of much use for anything :)
Step 4: Second Row
When the first row is done, ease the loops off the gauge. Then unhook the green loop from wherever you hanged it on and turn it over so the yarn is at the left end.
Now you are ready to start the second row. That can be the trickiest row since there is no real structure to the net yet. It is important to be sure to catch the next loop in turn from the first row, otherwise the net will be loop sided if it is supposed to be used as a rectangular net.
Start the second row basically in the same manner as you did before, but this time, you are not binding to the green loop, but to each of the loops of the previous row. Otherwise the knots are the same.
So, start by putting the yarn on top of the gauge, then catch the first loose loop from above and out through the loop to the right and tighten towards you, forming a new loop round the gauge. Tighten and put your thumb on it. Notice the third image, how the previous loop is twisted because how you entered the shuttle, and that crossing is important, otherwise the knot will not hold.
Then do the "throw over the thumb" thing, and stick the shuttle through the loop of the previous row in the way shown in image 4, that is upwards, above the twisted part of the loop and through the thrown loop. Tighten towards you.
Image 5 shows how a knot should look like.
Step 5: Third Row and Onwards
After you bound all the loops on the second row, ease them off the gauge and turn the green loop again to get the free yarn to the left.
From now on the binding will be a bit easier, because the net itself can help you a bit to find where to put the shuttle.
Start by making a loop over the gauge, under the gauge and catch the first loop from the previous row in the downward direction and out through the loop to the right. As you see the loops are now of full size and thus a bit easier to get through. Tighten towards you, hold with the thumb, and throw a loop over the thumb. As you can see in the second image, I am getting some help to open the right hole to stick the shuttle up through by pulling slightly on the next loop. As you see the yarn of that loop is crossed next to the gauge just like before because the loop was caught from above.
Now just keep doing this, consuming the loops until the end of the row. then ease off the loops, turn the net, and continue for as long as is needed.
Step 6: Finishing
Basically two things can be done with the net.
a) A conical landing net or a net bag for balls etc.
In this case you do the starting loop (the one I used green thread for) out of the same yarn as the net. Then you tie it off in a very tight circle that becomes the middle of the bag, and use the rest of it to tie together the sides of the net in a zig-zag fashion that makes holes similar to the ones you have bound. Then you can either thread a loop of thread along the opening to be able to close it as a bag, or you can mount it to some frame to make a landing net.
b) A rectangular net.
In this case you remove the starting thread(the green one) by just cutting it once and dragging it through the knots.
You then have a rectangular net, which can be finished by binding a frame of some thicker string along the top and bottom, and then attaching floats and weights to it to make it stand up in the water..
Variations: If you want a net that is circular to use as a throwing net, you can add loops with regular intervals by binding two loops in the same loop from the previous row. That way it can be finished like a net bag, but since you expand it each row, it will be able lo lie more flat out.
Slippery yarn: Some synthetic yarns can be very slippery and then the knots don't hold. That can be fixed by doing the part of the knot i called "Throw over the thumb and stick upwards" twice each time.
(Creds for the photos to my son, Folke.)