I wish I had a better answer, but for us, the best solution was simply lots of practice. My ds, now 14 and a freshman, had an IEP for expressive language in elementary and middle school. He had a ton of trouble articulating a sequence of events in a coherent manner. His speech pathologist first noticed it when she asked him to tell her how he walked home (from a school two blocks away from the home where he had always lived). It was the most convoluted set of directions she had ever heard - literally ten minutes of twists, turns, and steps in what should have been"something like, "go out the door, turn left, go straight, cross the street, I'm at my door.
She used that example a number of times over the years to explain his issues to teachers - while he could obviously GET home, he had no clue how to explain it, and the same was true with getting an answer/explaining how he got it. As part of his IEP, she worked with him over and over again (he was lucky enough to have the same SLP from K-5th) on sequencing, starting an explanation or paragraph with one idea/sentence, using graphic organizers, etc... In 4th grade, he had a wonderful teacher who let him sit with her at a table in the back of the room every single day when they had writers workshop. Other kids would come back and check in with her once during the time period, but he would be there at her side the entire time, checking in with her after every sentence, sometimes after every word. We and his teachers had to work with him on math as well - he was and is great at math, but showing his work was difficult. We would literally have to constantly stop him "okay, so you say you had 34 - there is no 34 in that problem, how did you get that number? Oh, you added the two races together, and subtracted the starting time first.. okay, let's step back and start over..." He was the kind of kid who spontaneously read (before he could speak well) and who seemed to know how to intuitively solve math problems, but I think that might have hurt him in that he never learned how to solve things step by step. He would look at problems and "know" the answer, and he wasn't quite sure how he knew. His motor planning skills were equally abysmal, lol - it was like the "learn how to do something step-by-step" gene was missing from him!
If it's any consolation, he has improved every single year and is rocking a pretty tough HS schedule (and by HS, everyone seems fine with him just solving the math problems!). Writing will never be his strong suite, but at least he can get through class without sitting with the teacher. That would probably not work as well in high school English.
On a funny/scary note, explaining your answers/showing your work is not a bad skill to hone. Ds's oldest sister had the same issues, and went to school before there was such a big emphasis on showing work. She was accused of cheating once on a college chemistry test. She apparently had the correct answer, but had skipped so many steps (it was not required to show work) that the professor felt it was impossible for her to have done it without cheating. She was called into a meeting, which DH also attended (we lived nearby and she was allowed to bring someone). Luckily, DH cut to the chase and just asked the professor to watch her solve some things, which did away with any concerns over cheating!
