Fill a large (or better extra-large) pot with the water and bring to a boil. If you do not own one that is large enough, split it up into two.
Toast each slice of bread two times. Make it really dark, as this will affect the color of the final product. And darker Kvass looks more appealing ;)
Once the water boils, remove the pot from your stove and add the raisins together with the toasted bread.
Cover it with a lid and let it rest for at least 8 hours.
Remove the toasted bread.
Mix sugar and yeast, add them to the pot and stir the whole mix.
Cover it up again for another 6-8 hours. Stir the mix every once in a while.
Strain the mixture into bottles with the help of a fine-mesh strainer or cheesecloth. Close the bottle caps loosely and refrigerate overnight.
You can seal the bottles tightly the morning after.