Long and low means having the horse going forward driving with his hind legs while reaching forward and downward with his neck. Do you know how a Stallion looks when he drives his mares forward? Or when a horse is excited about meeting another horse for the first time and they come toward each other? The way that the neck arches and reaches forward is the kind of shape you are looking for; lifting the base of the neck , reaching outward and letting the nose point down to the ground. You would probably have the horse's neck a bit further arching downward than the above examples, but my point was to emphasise the long, "telescoping" arch of the neck.
Also, it's very important that the horse be really stepping forward and under with his hind legs.
This sort of reaching down and stepping under is harder for Arabians due to their high neck placement, but they can do it. You will work on encouraging your horse to follow the bit forward and downward and to stay down there while trotting forward.
Also, trotting over ground poles, either on the lunge or mounted, helps develop the abdominal muscles and they are what lift the back into a good topline.