You need to make a new Sub Routine, not use an existing one.
I'll just go by this step by step for you.
Start by going to the Sub Routine tab before clicking on Create New, then insert your coding to disable the Special. It's very important to watch the Offset that's above the coding, as that will change when coding is added. When done, make sure the Offset in the text box matches the Offset above the coding. If you don't keep this in mind and change to the Sub Actions tab, your Sub Routine will be gone.
Next, you're going to make a Sub Routine command and use the Offset of the Sub Routine as its Pointer before copy + pasting it in every aerial Sub Action that you can act out of (i.e. Jumping, Falling, etc).
For aerial Sub Actions with an Allow Interrupt command in them, you would place the Sub Routine before the Allow Interrupt command.
After that, you're going to place a Bit Variable Clear command to clear the Bit you set when the character lands on the ground or gets hit.
To finish things off, you'll now place a Bit Variable Set command in the associated aerial Special Sub Action that you want to have a once per airtime usage. In this case, your Special is Neutral Special.
If you did everything correctly, the Special will be disabled until landing if used in the air.
For animation tips, start off by making simple actions and look at existing animations on how to go at your animation. You should also look for critique whenever possible.