Pro-tip: tell nobody you know personally about this.

So instead you must use baking soda?

Yes. Sodium bicarbonate. Make sure it doesn't have any flour in it.

Buy a tub of sodium bicarb and put it in an old shampoo bottle. Add water and mix - aiming for shampoo like viscosity.

Depends how much hair you have, your activity levels and where you live but start with using it once every couple days. You may end up at once / week, but depends on the above factors.

Use it like shampoo, but more vigorously.

Thanks!