Raising the Kh/pH will not do a lot for your guppies, Hardness and total dissolved solids would be more what they might need. What problems are you seeing with your guppies?
Baking soda dissolves and reacts quickly, 1 teaspoon will raise 30g by 1 dKH if I remember correctly. I would not mix it directly into your tank though, premix in a seperate container and add to the tank slowly.
To increase hardness (Won't change pH) add epsom salts and or Calcium in some form. CaSo4 or CaCL are both commonly available in the hobby. You could also use somethng like Seachem equalibriam.
And of course adding the crushed coral as earlier suggested would raise Both Kh and GH for you, and is easily controlled, stable, and not at all diffuclt or expensive. By using Crushed Coral (CaCo3) you don't get the negative of the sodium addition that you would get from baking soda.
Dave