Start by blowing up a balloon to about the size of a grapefruit, and rest it in a mug.
This will keep it steady while you're working.
Using PVA glue mixed half and half with water, paste on a layer of tissue paper or kitchen roll.
The idea is to go all around the balloon in this way, right up to the knot - but no further.
Do two or three layers.
When the tissue and glue is dry, you can carefully pop the balloon with some scissors.