How to manually create a mdf file for localdb to use?

Just use CREATE DATABASE statement

SqlConnection connection = new SqlConnection(@"server=(localdb)\v11.0");
using (connection)
{
    connection.Open();

    string sql = string.Format(@"
        CREATE DATABASE
            [Test]
        ON PRIMARY (
           NAME=Test_data,
           FILENAME = '{0}\Test_data.mdf'
        )
        LOG ON (
            NAME=Test_log,
            FILENAME = '{0}\Test_log.ldf'
        )",
        @"C:\Users\George"
    );

    SqlCommand command = new SqlCommand(sql, connection);
    command.ExecuteNonQuery();
}