Buy a skein of acrylic yarn. Choose a yarn that is dark in colour as it would be easier to find the eggs later. Soak the whole skein in boiling water for a while to sterilise it and to remove any residue dye. Soaking the skein in boiling water will also make it easier for the mop to sink in water.
Find a hardcover book, the length of which should be about the height of your breeding tank. Cut off a small strand of yarn and lay it across the width of the book.
Wind the yarn around the length of the book. Go round about a hundred times. Cut off the yarn from the skein.
Using the strand of yarn that was laid across the width of the book, tie a knot somewhere in the middle.
Flip the book over and cut the yarn somewhere in the middle.
What you get now is something like a mop.
Use another short strand of yarn to tie the mop somewhere near the top.
If you want the mop to float, tie a piece of cork on top of the mop. Your floating spawning mop is now complete.
Many thanks to William Shenefelt for the information on his website at: his website from which I learnt how to make a spawning mop.