Hiya.
You are right - when you get a 0 reading for ammonia, dose it back up to 2ppm and keep checking for nitrites too.
Every time the ammonia drops to 0, put some more in to a level of 2ppm. (Gotta keep those ammonia bacteria going).
I did a large water change inbetween as well to try and help reduce the nitrite level.
You will notice the nitrate level goes up as nitrites become 0. Then a very large water change should reduce the nitrate level.
When ammonia and nitrites read 0 iin 10/12 hours, you are there my friend!!