You can put the baking soda directly in the acid but SLOWLY. Just a small pinch at a time or it will bubble up and go everywhere.
NO NO NO. DO NOT ADD THE BAKING SODA TO THE ACID CONTAINER !!!
You will likely get a geyser of acid all over the place if you add
it to the bottle that has the acid in it. You must dilute the acid
down to around 10 percent in a large bucket and then slowly add the
baking soda or better yet sodium carbonate (washing soda) which has
been dissolved into water first.
James Binnion