The idea is to combine baking soda and water into a paste to apply to your hair. There are different approaches. I fill a small jar with baking soda and then add enough water until it forms a paste that I can easily scoop out with my hands and lather into my hair. You could use an old shampoo bottle or cup and a higher ratio of water for a more liquid-y solution that is easy to squeeze onto your hair. Some people also like to do a rinse with white or apple cider vinegar afterward as a conditioner, but my hair feels the same whether I use vinegar or not, so I don't.
If you miss the scents of shampoo, you could add something yummy smelling, like a drop of lavender essential oil, cinnamon sticks or vanilla extract, to either the baking soda or vinegar, but I go plain. One jar lasts about a couple weeks, and since baking soda is readily available for about $2 a box, it is much cheaper than shampoo. This also helps me reduce my use of plastic.