Adding Cabosil, or the thickener of your choice, is easy but you can not make it too thick. It needs to drool off the mixing stick in a steady stream. If it goes drip, drip, drip off the mixing stick it's probably too thick.
Remember this slightly thickened resin has to fill a hole and let air bubbles escape leaving behind no voids. If you make it too thick you will have bubbles and voids. Not too thin and not to thick..