A full jug of gunge poured carefull down inside the front of your trousers, taking care not to spill any on the outside, and pouring dead-centre so the gunge flows down to the crotch of the trousers and divides, some going down each leg.
And what a lovely game to play on the first day of the new year, thanks for that!
Grab a towel. Put it between your legs. Grab one end in the front. With your other hand above your head, grab the other end on the back. Pull up on the towel to give yourself a wedgie and making the gunge squish up in your trousers. Pull the towel back and forth to make your gunge filled trousers smoosh around.