Also try to imagine if you can sharing the down command with whatever special move you're doing. Instead of doing down 4 and then down back 2 for acid hand try down 4 and as he kicks do back 2. Etc.
All I can say is practice putting it in as one combo like bookburning said. That's why the negative edge is there so that if you do it quickly, the 2nd Down input goes off and tells the game that you want to do the "fireball" input.