Put some sandpaper on a table and rub the bottom on it until it doesn't wobble.The resin makes the base juuuuuust a bit wobbly
If you want to do a better job, rub chalk on paper to coat it, put it on a flat surface, then rub the base of the cup along that. Turn the cup over and you'll have chalk on the "high" spots of resin. A little hand sanding in only those areas will better preserve the cup. Repeat until the chalk only appears on three spots, or over the surface, so it no longer wobbles.