Throw out what you've been told about parking yourself cross-legged for twenty minutes and aiming for a "blank mind." That sentence alone was designed by someone whose brain has only barely a few tabs running at once, google meditate.
The ADHD brain feeds on dopamine and novelty and and a stream of sensory information — so the opposite of those things (stillness, hush, repetition) when requested of us, would in most people cause them to feel not good enough, like everyone else would; but also feel like a massive letdown. Which, again, does not come from willpower — it comes from using it the wrong way. Two minutes is the target.
Do not try for ten.
Commit to just two minutes; which, in the first 30 seconds of which, you are going to lose it in some kind of spectacular way anyway; so with all due respect — the less runway you have when it first zooms away, the less time it has to get away from you,and you can pull it back! Success is simply coming back —and this is your entire rep. By doing the rep for 50 occasions throughout two minutes —you would most certainly generate more focus muscle than someone who just quietly sat for twenty minutes on the floor —totally disconnected from any thought.
Movement works wonders; meditative walking, free-form dancing, even wandering the house; or any kind of movement while trying to meditate... Whatever works! Attempt pacing the house or the room with a fidget object and just pay attention to your breathing when it wouldn't be a good situation at all —if anyone in the world were to witness your behavior! Surprisingly enough... "tidy up the kitchen" meditation (running water, hands busy and mind in its body) is often times how the most present and intelligent minds actually find bliss with meditation instead of a flawlessly posed, yoga-poster like, meditative bliss pose.
Noise can be another rescue where total stillness becomes almost unbearable in an ADHD brain. Play your favorite guided sessions, binaural beats, or even just the tick of a clock in your room and your brain will handle the rest by fixating on whatever thing it is listening to!
Timing of this meditation session during your day matters too... Should you have prescription medication that you take at some point daily then this is frequently a much better time than right when you wake up, when your brain is far from switched on.