It isn't essential to memorize the words. I try to, but don't always have the time. just make sure you practice so you can read them without hesitating or stumbling over them. this keeps the flow of the spell runnig smoothly.
And if you should make a mistake,(and even the best of us do from time to time) don't worry about it. just laff it off n keep going. The God/dess has an excellent sense of humor.