While it can be easiest to use a food processor since you can then pulse as you add the flours, you may want to use a blender initially if your food processor is not that good at breaking up the spinach. It's OK to have little chunks of spinach, but the pasta will be smoother if you can get it really fine (and it'll be a more even green and easier to cut into linguine etc). If you use a blender, you can then either transfer back to food processor or, to have less to clean up, mix by hand. Place the flour on your work surface in a pile, make a well in the middle and then pour the egg-spinach mixture into it. Use a fork or your hands to gradually incorporate the flour into the egg mixture.