You don’t actually want to practice it in a L shape like Kelter said, because in real game play, you will in fact make a smooth quarter circle. If you do a L shape, its 2 seperate movements, which won’t cut it when you need to pull it off on reaction in a split second. His heart is in the right place, but it seems to me that he’s trying to over think it. Afterall, we all do quarter circles, not quarter squares.
just do the damn motion really really slow and make sure its right. then do it a little faster. keep speeding up bit by bit till your up to speed.
like i said, slow down. I wasn’t trolling, my answer was short, but thats all there is to it. just slow down.
you don’t need a 4 page essay on doing it slowly right? 
this is a basic principle in all of life. when learning a new movement, either in video games, sports, martial arts, playing guitar, or anything, you start slow making sure its perfect and slowly speed up.
Its a quarter circle. None of us can tell you why your missing it unless you post a video of your self doing the motion. But thats not nessary. Just practice slowly and you’ll get it. Your probably trying to do it really fast, way too tense and spazzy on the stick. Common among new players.
On a side note, you also don’t actualy need to hit the diagonal. down then straight to forward will also throw a hadouken. Thats the actual input the game is looking for. The diagonal part is mentioned just to make it easier, as a quarter circle is faster and more accurate with a joystick then down then forward.
But if your playing on a pad, skipping the diagonal, and just going down then forward is better. You just rock the D-Pad between the 2 positions.