To grind flour in a Blendtec, start by selecting the correct jar for grinding grains, such as the Twister jar or the FourSide jar with the vented Gripper lid. Make sure the jar is securely attached to the Blendtec base. Next, add your desired grains to the jar and close the lid. Select the manual speed button and gradually increase the speed to high. Blend for about 50-60 seconds or until the grains have been ground to the desired texture. Stop the Blendtec and remove the lid to transfer the flour to a storage container.