Step 2: Combining baking soda with vinegar
Start by pouring all but one cup of the white vinegar into the boiling container (5.5 Qt or larger). (See note about acids below...)
Then, carefully add baking soda to the vinegar, small amounts at a time (no more than a tablespoon). Sprinkle it over the vinegar - don't just dump it in. If you add too much baking soda too quickly, the foam from the reaction may overflow your container.
Stir gently after each addition of baking soda to ensure no unreacted bicarbonate remains.
After adding about a half box of baking soda, you should notice the reaction starting to slow down. At this point you may want to reduce the amount of soda you're adding each time to a teaspoon. When a teaspoon (or less) of baking soda sprinkled over the solution no longer bubbles instantly but bubbles very sluggishly as it sinks into the liquid, it's time to stop. In my experience, it's taken roughly 12 oz of baking soda to get to this point (about 3/4 of a 16 oz box).
Finally, add the retained cup of vinegar to the liquid. Since judging the stopping point can be difficult to eyeball, I've found it helpful to withhold a small amount of vinegar and add it after the baking soda reaction almost stops. This way, we err on the side of excess unreacted acetic acid (which will largely boil off) rather than excess unreacted sodium bicarbonate (which will not, and can interfere with crystallization).
(Note on acids: yes yes, I know, we're all taught in science class that you never add X to acid, you add acid to X... Good! Brownie points for remembering that! This stuff, however, is a 5% solution of acetic acid, and personally, I'm not too concerned about burning my skin off. If you're too uncoordinated to keep it out of your eyes, wear goggles.)