When we think in general terms not worrying about the how or when we are in a more allowing state of mind, because we are letting the Universe determine the specifics. So Abraham uses the analogy of the Grid, being the general framework or structure, and the process of the Grid filling in, is the process of the Universe finding the specific details. When that happens you get the manifestation or signs of the manifestation being on the way.
You do not need to imagine the Grid in meditation etc, just keep yourself general. For example if you say, I want a new car. That is your grid or a big square. Then within this square are smaller squares, there is a square for color, and a square for power, and square for seats, .... and many squares. These are the specifics. If it makes you feel good to think about the specifics then go ahead and do so as this will speed up the manifestation. But if the process of thinking about the details takes you into negative territory and you start thinking thoughts like "a car with leather seats is going to be so expensive where will I come up with the money for it" - now you are getting too specific. Abraham says leave the specifics to the Universe. You don't really need to put all the details into words, the Universe already knows what makes you happy. Let the universe do the work of coming up with the specifics. Allow the Grid to fill in rather than try to fill it in yourself.
Does that make sense?