how do you make baking soda into a tablet?

Try a tube, a metal object that fits snuggly in the tube, and a hammer. but in the powder, put in the metal object, and hammer it down. BE CAREFUL!
That's a quality diagram!
The old way was to use a press tool and make each one by hand. A die was filled to the appropriate depth with powder and a pin PUSHED into the die compressed the tablet.  You can't HIT the pin, the tablet collapses.

You need a very fine powder, and a little bit of moisture helps bind things together. Pharmacists used to also add a tiny bit of sugar.