The wobbling tool
After a little experimentation I cut a length of thin plywood (thick card or plastic will also work) a little wider than the platform. This will form the spreading tool, which will help to bed the plaster in and remove any bubbles. The trick is to wobble the spreader from side to side, rather than simply dragging it. The card edging stone might start to shred a little with the moisture if worked too aggressively. If this happens, don’t worry. Once the plaster is fully dry you can remove any shredded areas with the sandpaper.