What rules are to be observed for the right understanding of the ten commandments?

Westminster Larger Catechism

Q. 99. What rules are to be observed for the right understanding of the ten commandments?
A. For the right understanding of the ten commandments, these rules are to be observed:

1. That the law is perfect, and bindeth everyone to full conformity in the whole man unto the righteousness thereof, and unto entire obedience forever; so as to require the utmost perfection of every duty, and to forbid the least degree of every sin.[422]

2. That it is spiritual, and so reacheth the understanding, will, affections, and all other powers of the soul; as well as words, works, and gestures.[423]

3. That one and the same thing, in divers respects, is required or forbidden in several commandments.[424]

4. That as, where a duty is commanded, the contrary sin is forbidden;[425] and, where a sin is forbidden, the contrary duty is commanded:[426] so, where a promise is annexed, the contrary threatening is included;[427] and, where a threatening is annexed, the contrary promise is included.[428]

5. That what God forbids, is at no time to be done;[429] what he commands, is always our duty;[430] and yet every particular duty is not to be done at all times.[431]

6. That under one sin or duty, all of the same kind are forbidden or commanded; together with all the causes, means, occasions, and appearances thereof, and provocations thereunto.[432]

7. That what is forbidden or commanded to ourselves, we are bound, according to our places to endeavour that it may be avoided or performed by others, according to the duty of their places.[433]

8. That in what is commanded to others, we are bound, according to our places and callings, to be helpful to them;[434] and to take heed of partaking with others in what is forbidden them.[435]

Source: Daily Westminster, Jul 5 | Daily Westminster (wordpress.com)