How do I apply for an NRCS grant for stock water?

I would start by visiting the NRCS official website, How to Apply | Natural Resources Conservation Service. There they show a 5 step process that includes:

Step 1: Make a Plan
Step 2: Start an Application
Step 3: Check your Eligibility
Step 4: Rank your Application
Step 5: Implement your Plan

We recommend reviewing these and then visiting with your local NRCS office to ask any questions or start the process. If you don’t already have a farm number, you’ll need one from your local FSA office, which is usually in the same building.