John Gill Commentary Deuteronomy 12:32

"What thing soever I command you, that shall ye observe to do: thou shalt not add thereto, nor diminish from it." — Deuteronomy 12:32 (ASV)

Whatever thing I command you, observe to do it In the manner it is commanded and directed to; the laws of God, both as to matter and manner, were to be obeyed just as they were delivered: you shall not add to it, nor diminish from it; neither add any customs and rites of the Heathens to them, nor neglect anything enjoined on them, see (Proverbs 30:6) (Revelation 22:18Revelation 22:19) .