It only takes a few minutes.
Pour in a clean, dry bowl:
1 cup of school glue (clear or PVA)
1 teaspoon of bicarbonate of soda
Mix them together.
Add a little of gel food colouring (optional).
Mix it into bicarb and glue mixture until it has fully integrated.
Add 1 tablespoon of contact lens solution (that contains boric acid).
Mix with a spoon until it starts to become stringy.
Than take it out of the bowl and knead it between your hands at least 20 seconds till it will firm up and stick only to itself, becoming elastic and super stretchy.
If it’s still too sticky then add just a few more drops of contact lens solution until it’s just right. Be careful as too much will result in it becoming brittle and snapping.
You can add some glitter, colourful beads or sequins to make it even more pretty.