I just want to add a quick disclaimer that I have not been diagnosed with ADHD myself, but I have worked with several clients who struggled with this, and done some research online to compile a few tips you might be able to implement in your writing! (I will also link to some articles I read at the end of this post).

1. Turn off the internet This popped up as one of the most common pieces of advice for ADHD writers. Eliminating distractions can be hard, but turning off your wifi altogether might help with at least a few of them. Train yourself to disconnect when it’s writing time, and see whether this affects your productivity positively!
2. Delay the research This can be really difficult if you’re writing historical fiction, or anything set in an area that requires some level of prior knowledge to write about. But, wherever possible, if you can save yourself pausing to research whether a certain item or word is plausible for you to use in that specific moment, then you should. Leave it, make a quick note that that’s something to research later, and keep going.
3. Test out radical changes Something that came up as a common downfall was losing interest halfway through the project, or dropping it after the first chapter. If you find yourself losing interest in the project, maybe try out making a radical change to a part of your plot, one that might inspire you for the story once again. Of course, make sure that this change fits in with the rest of your story as you begin to implement it.
4. Succinct & clear goals This is something I recommend to any writer or artist of any kind! Keep your goals as clear and small as possible. The more specific you are with a goal, the less overwhelming it will feel. Think of breaking down a big goal of “I’ll finish my book this year” into “I’ll finish 5 chapters this month” or even “1 chapter this week.” Go as small as is comfortable for you!
5. A fine line between plotting & exploring This is an issue that comes up very often for writers who have a tendency to get bored with their projects if they don’t keep some element of surprise to it. I’d recommend finding a good balance between how much you plot out, and how much you allow yourself to discover along the way. Find your most important plot points, and let your creativity fill in the gaps!
6. Tools to help you out Stay on task - browser extension that checks in on you at timely intervals to make sure you’re working on your chosen task Ommwriter - atmospheric simple writing program, with typewriting sounds & ambience Stayfocusd - browser extension to block time-wasting websites Nanowrimo.org - community challenges, self-set goals, achievement badges, progress tracking The Emotion Thesaurus: A Writer's Guide to Character Expression - a book to help with expressing emotion in a show-not-tell way

Further reading from some ADHD fiction writers themselves:

Feel free to share your own advice in the comments below to help out other authors!

• shower. not a bath, a shower. use water as hot or cold as u like. u dont even need to wash. just get in under the water and let it run over you for a while. sit on the floor if you gotta.
• moisturize everything. use whatever lotion u like. unscented? dollar store lotion? fancy ass 48 hour lotion that makes u smell like a field of wildflowers? use whatever you want, and use it all over. 
• put on clean, comfortable clothes. 
• put on ur favorite underwear. cute black lacy panties? those ridiculous boxers u bought last christmas with candy cane hearts on the butt? put em on.
• drink cold water. use ice. if u want, add some mint or lemon for an extra boost.
• clean something. doesn’t have to be anything big. organize one drawer of ur desk. wash five dirty dishes. do a load of laundry. scrub the bathroom sink. 
• blast music. listen to something upbeat and dancey and loud, something that’s got lots of energy. sing to it, dance to it, even if you suck at both.
• make food. don’t just grab a granola bar to munch. take the time and make food. even if it’s ramen. add something special to it, like a hard boiled egg or some veggies. prepare food, it tastes way better, and you’ll feel like you accomplished something. 
• make something. write a short story or a poem, draw a picture, color a picture, fold origami, crochet or knit, sculpt something out of clay, anything artistic. even if you don’t think you’re good at it.
• go outside. take a walk. sit in the grass. look at the clouds. smell flowers. put your hands in the dirt and feel the soil against your skin.
• call someone. call a loved one, a friend, a family member, call a chat service if you have no one else to call. talk to a stranger on the street. have a conversation and listen to someone’s voice. if you can’t, text or email or whatever, just have some social interaction with another person. even if you don’t say much, listen to them.
• cuddle your pets if you have them/can cuddle them. take pictures of them. talk to them. tell them how u feel, about your favorite movie, a new game coming out.