To get it out of the way first, a freshwater dip can't, and won't, kill off everything bad. The very nature of burrowing parasites prevents that. The fish would be dead before all the parasites are. A freshwater dip can provide immediate relief, temporarily, to a fish covered in Ich/flukes/etc, but it doesn't completely cure anything. And if you're planning on properly quarantining your fish, I suggest against it. In QT, you want to know if your fish has a disease. FW dip a fish, and knocking off the outer-most parasites might cause you to miss them.
Baking soda can be used. All pure baking soda is, is sodium bicarbonate. pH buffers sold specifically for aquarium use, are more efficent, and as Flower noted, you'll know about how much to add. For temperature, you the temperature no more then the tank temp. A little cooler is fine, and if you're unsure of the temp, lean colder, instead of warmer.
What no FW dip should be performed without, is Methylene blue. Methylene blue provides extra oxygen in the water for fish, and aids in calming the fish down. Methylene Blue is commonly available.
Activity of the fish, at first the fish will be quite hyper and darty. Make sure you are watching, and if the container is not too deep, having something over it to make sure they don't jump out. After a few minutes, the fish should calm down, and most fish will sink to the bottom and lay on their side. That is normal. Some fish will also change colors, change coloring patterns, or erect all of their fins/spines. All normal. If unsure, the fish can be gently poked (more like bumped) and they should react. What you want to watch is the fish breathing. After they've calmed down, they should be breathing normally, or slightly rapid. If they stop breathing, or are breathing extremely fast, the dip should be stopped. A dip can go up to an hour, but a normal dip is 10-30 minutes.