Eric, my 14 year old non-verbal son with autism has no verbal speech. At some point, he was diagnosed with a very low IQ and supposedly with a mentality of a toddler. However, any one who spends 5 minutes with him begs to differ. Over the years, he has often surprised us by how smart he is and his capacity of learning things independently like how to use Google on the computer and get to Youtube on my cell phone.
I try to teach my son by providing frequent opportunities throughout the day in his natural environment. Furthermore, I tend to narrate as often as possible: what he is doing (Eric is watching Toy Story), what I'm doing (Mommy is washing the dishes), what someone else is doing (the man is mowing the lawn), or just noting a certain event (it is raining).
For example: When I notice my son is rubbing his eyes and he is sleepy, I would say "you look sleepy..It is time to go to bed". If I notice that his hands are dirty after playing basketball, I would say "your hands are dirty...go wash them". These particular commentaries will progress into "you hands are dirty..what do you need to do?", to simply making an observation "your hands are dirty" with a non-verbal cue pointing him to the sink to wash his hands. Eventually, he would be noticing on his own that his hands are dirty and proceeds to wash them independently . These examples of incidental teaching mixed with narrating, modeling, intraverbal style of communicating, using prompting/fading, all in the natural environment, could also be reinforced using structured teaching.
I designed an activity called "what do you do when...."?. If your child can read then you can just use written language. Otherwise, use pictures. I use one set of index cards for the "What to do" and another set of cards for the "when". Examples of "what to do" answers would be "wash them", "go to bed", "Turn up the volume". The corresponding "what to do" questions would be "when your hands are dirty", "when you are sleepy", and "when you can't hear the TV".
My son proceeds to match the answers to the corresponding questions. This is particularly helpful if the child is a visual learner and seeing the actions in words helps reaffirm the concepts learned incidentally.
We often concentrate on teaching skills in isolation. For example: teaching prepositions such as "in", "on top of", "over" and so forth. It is definitely good practice to teach in a structured, controlled environment with trials conducted in a systematic manner. However, it is as important to teach these skills within the context of every day use. For example: When teaching "in", you could tell the child to put the orange peels "in" in the trash, get "in" the bath tub", pour the milk "over" the cereal, stand "behind" me in the line, get "inside" the car, etc.
Write down these prepositions with a sharpie (BLACK or RED) on index card and flash them and point at them while interacting with the child when giving him such directions. It would also help point to the word to emphasize the word/preposition being taught. It is always a good idea to have a visual support and it is a good step to teach pre-reading/reading skills.