I've read about two methods: Johnson's paste wax and clear vinyl sheeting bought at the fabric store or tablecloth.
I use the clear vinyl sheeting on my old Singer which doesn't have its own cabinet, so I build up my dining room table with books and lay a cardboard cutting board with a hole for the machine on top. The vinyl goes over the bed of the machine and the cardboard. I use the paste wax on my vintage Bernina and the formica of its cabinet.
Both methods work well for creating a slippery surface, however, I find the paste wax more convenient for bobbin changes.