Cornstarch and tapioca flour both act as thickening agents. If you have to use cornstarch in place of tapioca in a recipe, remember that cornstarch will take longer to dissolve and to thicken when mixed with other ingredients. Tapioca flour produces a glossier sheen than cornstarch, so if you use the latter for a pie filling like lemon meringue, the filling won't appear as shiny.
Assess how much tapioca flour is needed for a recipe. For example, you're baking a pie that needs 2 tbsp. of tapioca flour.
Divide the amount of tapioca flour called for by 2. (You only need half the amount of cornstarch when it's being used in place of tapioca.)
Add 1 tbsp. of cornstarch to the ingredients, and follow the rest of the recipe as directed.