The desired SQL statements

As usual, the easiest way to build a template is to start with a copy of the output you want. Here is the procedure for the Person.AddressType table in the AdventureWorks sample database:

CREATE OR ALTER PROCEDURE Person.GetAddressTypeJson
WITH EXECUTE AS CALLER
AS
BEGIN
    SET NOCOUNT ON;
    SET TRANSACTION ISOLATION LEVEL READ COMMITTED;

    SELECT
        [AddressTypeID],
        [Name],
        [rowguid],
        [ModifiedDate]
    FROM
        [Person].[AddressType]
    FOR JSON PATH, ROOT('AddressType');
END
GO

Running EXEC Person.GetAddressTypeJson returns the whole table as one JSON document, shaped like this (values shortened):

{"AddressType":[{"AddressTypeID":1,"Name":"Billing","rowguid":"…","ModifiedDate":"…"},{"AddressTypeID":2,…},…]}

The statement contains two kinds of variable information. Everything else in the template is static text.

Kind of information	Where it appears
Values the user chooses from a short list	The EXECUTE AS value (CALLER), the isolation level (READ COMMITTED) and the FOR JSON mode (PATH).
Values CodeSmith Generator reads from the database once the user picks a table	The schema (Person) and table name (AddressType) in the procedure name, the source table and the JSON root; and the column list.

About the SQL

You don't need to know every option in detail to follow this tutorial, but here's what each part does:

Part	What it does
CREATE OR ALTER PROCEDURE	Creates the procedure, or replaces it if it already exists, so the script can be run again after the table changes.
WITH EXECUTE AS	The security context the procedure runs in: CALLER (the user who runs it), OWNER (the procedure's owner) or SELF (the user who created or last altered it).
SET NOCOUNT ON	Stops SQL Server sending a "rows affected" message, which some clients mistake for a result.
SET TRANSACTION ISOLATION LEVEL	How the query's reads interact with other transactions: READ UNCOMMITTED, READ COMMITTED, REPEATABLE READ, SNAPSHOT or SERIALIZABLE. SNAPSHOT needs ALLOW_SNAPSHOT_ISOLATION turned on for the database.
FOR JSON PATH	Returns the result as JSON, with the column names as property names. FOR JSON AUTO shapes the output from the tables in the query instead.
ROOT('AddressType')	Wraps the rows in a single object with this property name.

Next: Create the template