How does a person make their "calling and election sure" (see 2 Pet 2:10, below)?

The verse you provided, means that God does elect. He has not given that information to us as to who is elect and who is not. We know who's who by their fruits, while God searches the heart. And the page you provided exhorts the elect by showing what brings the Christian into fellowship with God. In other words, don't think you can ever, ever just give lip service to God while living the life of a heathen. If God has removed your heart of stone and given you a heart of flesh, here's how to use your heart of flesh.