Spread the starch in a thin layer (0.3-0.5 cm) on aluminium plate or foil and dry at a temperature lesser than 50°C preferably in a fully ventilated oven.
I dry starch under vacuum (e.g. in a speedvac or a desiccator) over night. Drying the starch at higher temperatures risks that the native structure is weakened and starch granules might dissolve. However, if that is not important for you, you may also dry the starch in a drying cabinet at 50 °C