This is what we're aiming for - it stops your yarn cake falling apart or getting tangled in your keys in your bag.
First choose your yarn - I tend to use leftovers of cotton 4-pky or dk. You'll need about 15g. Choose a hook suitable for the thickness of the yarn. In the photos I used DK cotton and a 4mm hook.
Make a slipknot and chain 10. This base chain decides the width of your finished net so if you are using one of those mega ball winders then make this number bigger.
Chain 5 (first treble plus 3ch), 1 tr into 5th chain from end
Chain 2 skip 1 and treble into next chain. Repeat 3 more times. DON'T TURN THE WORK!
Into same stitch ch3 treble, ch3 treble. This effectively turns the corner and enables you to do the same loopy thing up the other side of your starter chain.
Repeat to match first side of chain (where made first treble). Ch3 and join to 2nd chain of first treble.
At this point it should look something like a freaked out sea cucumber, a la picture below.
That's your base done.
Slip stitch across 2 chains - you're now on the peak of one of the little loops. Ch4 and dc into next space. You now repeat this round and round in a spiral.
At the end of the first round you might be tempted to join the end of the round with the beginning at the place the hook is pointing to below.
Instead make your last ch4 work a little harder and just dc into the next loop, where the yellow arrow is.
Keep going round until the cake holder is a bit shorter than you want it to be (about 9cm unstretched/7 loops high for me with my low tech average size ball winder). It should look a bit flacid and useless, like the pic below.
Now you're going to sort out the top so it can be gathered tighter to keep the yarn cake in it.
From wherever you are on the brim ch3 (first treble) *ch1 skip 1 1tr into next stitch. Repeat from * to end. Join this round to the 3rd chain of your first 3ch. You've created some little gaps like the pillars that held up aquaducts in ancient rome.
The pattern is on Ravelry as 'Yarn Cake Keeper'. Let me know if you spot any mistakes!