If you get the special on the ground you pressed the button too soon.
If you get a normal early in the air you either didn't input the required directions or didn't input them fast enough.
If you get a normal on the ground your fingers aren't cooperating with your brain. Take at least a 3 second break before trying again. This only happens if you press the button too soon for the ground version or you completely botch it.
IIRC getting superjump means you have to delay the button press even longer before getting airborne which makes it harder. Doing the hold 2 before doing the rest of the motion helps to not get a superjump but has no other technical effect, but the delay can help you with timing/concentration.
You don't have to do it slow/fast. You can input the directions for the special at any speed you want that would normally work. After the last input you need to jump, wait for jump startup, then press an attack button in the time you normally have for one input. Doing it slow then speeding up at the end works, but you could also do the first part very fast then need to slow down for the end. For newcomers to tking I recommend doing all of the directions fast, including jump, then slowing down for the button press by a lot. The first several attempts usually result in a lot of jump normals due to a late button press, just keep pressing it a little bit sooner and you'll find the timing. While it's not important in most combos this has the added benefit of allowing you to get it out sooner when you need to react to something.