What to do while waiting

You can start learning grammar as soon as you want. That’s just their recommendation not some ironclad truth. If you like videos there’s ones like Japanese Ammo with Misa, or you can use references like Imabi, Maggie Sensei, etc. Or if you prefer a textbook you can always pick up something like Genki or Minna no Nihongo. There’s a whole lot of choices on the spectrum from the free to the paid.

Edit to add:

This might be a good place to start: