This build is for a standard German rules chain, but could easily be altered slightly to make different style chains should the mood strike you.
You will need:
Plastic Chain - About 290cm
Foam Ball - 20cm Diameter is perfect
ParaCord - A lot
Tape - Strong fibreglass reinforced tape but not gorilla tape, that's to hard and will cause your chain to hit people like a sack of bricks.
Cylindrical Padding - I used a pool noodle, but ideally I would have used high density pipe insulation, but had none available.
Rope for the handle - I used a fabric handle from a reusable shopping bag as they're cheap and very strong.
A good knife.
Step 1: Threading the Para Cord Through the Chain.
Thread the paracord all the way though the chain, up one link and down the next. Tying it off to both end links. The cord has to be completely taut when the chain is fully stretched out but can't be so taut as to prevent the chain from reaching full extension.
The paracord is extremely important, as chains will break. Much more than you might expect. And when the plastic shatters the cord will prevent the chain becoming a missile.
It also improves the way the chain plays, it becomes much more responsive to quickly whipping up to speed and is more responsive to flicks and movements mid rotation.
Take one extra link that is separate from the chain, and attach it the end by tying it it all together with paracord. See the picture. Again, this is done to make any break or failure modular and easier to repair. By putting the plastic link in to take the force involved in chaining, your giving yourself and easier repair job than if the harness breaks.
Step 2: Making the Harness for the Ball.
The ball of the chain is going to sit in a harness or net that will be made from more paracord (see why I said you'd need a lot?).
Start off by making two small circles of cord, about 5cm diameter.
Then cut about 8 lengths of cord about the circumference of the ball.
Tie the lengths of cord to one of the circles so as you've made what appears be a cute little octopus.
Put the ball in the middle and the second circle of cord on top and tie the legs of the octopus to the top circle.
Now the ball should be in a little net, spread the cords out so as they are evenly spaced around the ball.
Take one more length of cord and weave it around the middle of the ball, wrapping it around each of the vertical cords it comes to until you get back to where it started. Tie it off to itself.
Take the lighter and melt your knots. Para cord is man made, all polyester and essentially just plastic. Melt your knots and they become stronger. This is very important, if like me, you are shit at knots.
Step 3: Finishing the Ball.
Take some of the flat padding and cut out two squares. These should be big enough to cover the mess of knots at the top and bottom of the ball.
Tape them over both ends but leave part of the top circle exposed as you'll need to tie the chain to it.
Step 4: Attaching the Handle.
Take the handle, and weave the ends of it though two chain links and tie it off. Get someone who is good at knots to help if you need to. Once its tied off good and tight, tape it up. Use as much tape as it takes to stop it having any wobble from the chain.
Then tape around the handle itself so you end up with a nice smooth finish.
The loop should just fit over your wrist, but I find this style plays best when its help under your fingers and not your thumb as it allows for an extra few inches of reach. Sometimes it matters.
Step 5: Attaching the Ball to the Chain.
Take the extra chain link you added to the end of the chain and tie a good length of para cord to it, maybe about 50cm.
Once its tied on, wrap it around the link multiply times. Then bring it down to the top circle of the ball and tie it onto it in more than one place, wrapping it around the circle in multiply places.
Finish it by bringing it back to the link and wrap it around multiply times again and then tie it off. This should be as taut as possible. The closer the link is to the ball the better.
Step 6: Padding the Chain.
Cut the pool noodle into pieces. Longer sections of padding will help prevent the chain from getting wrapped around a staff. But make sure to put a small section in between the larger ones to ensure it can still swing smoothly.
Tape the padding up but only tape to the chain itself at its ends, if you tape in along its whole length it will make the chain to rigid.
I used 4 sections of padding. A small right at the ball to cover the two links that are tied together. A long one, a short one, and a long in that order. I've found this gives the best balance between not being able to get wrapped around a staff when being blocked and being able to swing around a block to hit the player.
I have a small piece of much thinner padding just above this that isn't tapped to the core and can rotate and slide freely. This works as a place to hold the chain while swinging and, I find at least, makes it easier to use.
Lastly, pad about 15cm just below the handle. It may seem odd to pad that end of the chain, but having a more rigid end will make it much easier to spin back up quickly and make it more responsive will spinning slow.
Step 7: Taping Any Remaining Parts.
Tape over any exposed padding and cord on the ball. Don't tape on too tightly as this will make the ball hit very hard, just cover the cord net you made.
Step 8: Strike Fear Into Your Enemies.
Go out and practice, chain is tough to learn but so much fun to play.
Should you wish to add different style handles, just alter that step. An example is a baton weapon that you can also use to strike opponents. If your ruleset allows such things, there's a picture of a chain with one of those above.