I always think that it is good to break large tasks requiring long sustained attention, into small bits, taking them one at a time and not worry about the end goal. Not only does it make it easier, but it removes one of the things that makes A.D.D. worse, which is pressure and the fear of failing. Take away that strain and you only have the problem of the A.D.D. to deal with. Also go for walks and think problems though while relaxed and employed, so that the outer mind is busy and can not distract the inner. Walking has been proved to help concentration.

Make a game of trying to remember things by finding or inventing logical or historical narratives. If for example you are having difficulty with spelling and you can not spell, psychological. Look up and learn the Greek legend of the demi-goddess Psyche, and then add a story you make up yourself about an Irish philosopher called O'Logical and you have it. Those sort of ways of working are used by people who do memory stunts, but they work just as well with A.D. D. as well. And remember that mnemonics that you research and make up for yourself, work better than the ones that other people give you.