Yes, beware of You Tube 'tutorials'.
My missus wanted me to make a plaster-of-paris slab for drying out surplus wet pottery clay, and we wasted spent 3/4 of an hour watching some famous potter failing to make one. He made up an elaborate frame to contain the plaster, then couldn't get the slab off the car window he'd used as a base, and just gave up.
We used an old cat-tray lined with polythene. That was 5 years ago and the slab still works well.