Working with corn starch can be tricky especially if you are adding it as a thickening agent into hot liquid as is the case with this pudding. This is because corn starch tends to get lumpy when added into hot liquid if not stirred quickly. The best way to handle corn starch is to sift it first before adding into any liquid so that there are no lumps. Secondly, it also helps to mix it with ingredients like sugar (if the recipe calls for sugar), so that it is dispersed and does not get a chance to lump. And the final way is to dilute it with a small amount of liquid, so that it is disperses in the liquid and does not lump up when added to the hot liquid.