Yes, that's what they meant (or that's what you should do. rinse until as clear as possible IMO). The plate will break the stream of water and keep it from disturbing the substrate.When you said "give it a good rinse", you mean to put the substrate in a bucket, fill it with water, rinse it, drain it and repeat till it's a bit clear, right? Also, why should I place a large plate?
What substrate are you adding?