Sure! Here’s how you can fill in the blanks with prepositions for the passage:

1. **in** - Fill a large pot two-thirds (1) **in** water...
2. **to** - ...and bring this (2) **to** a boil on the stove.

The complete sentences would read:
"There are many ways to cook and eat corn, but boiling corn is a good way to make it tasty and healthy. This is how you can do it. Fill a large pot two-thirds **in** water, stir in 1/2 tablespoon of salt, and bring this to a boil on the stove."