Having been delivered from the Law of sin which is in one's fleshly members through Jesus Christ, we then serve God's Law with our mind/heart (I shall write my Law upon their heart). But, with the flesh, we serve that Law of sin (and death). (Rom 7:23-25)
Seeing that there are two sets of Laws being spoken about, Law of sin and God's Law, and in verse 22, we are told that Paul delights in God's Law, and in verse 23, we see that the Law of sin wars against that Law which God placed in our mind/heart, then we can conclude that the Law of sin is the Law which we have been liberated from and brought under the Law of the Spirit sent by Christ known as God's Law.
Have we been liberated from God's Laws which are fulfilled by Jesus' command: Love....? Does not the Spirit have law?
You asked how do we establish the Law? We don't establish 'the Law', neither do we 'make void the Law' (KJV inserted the definite article in both instances). With that said, the question in Rom 3:31 is, 'Do we then make void law through faith? May it not be so. Yes, we establish law'. That is, we Christian are still under law. Specifically, that Law from the Spirit which walks us in love and by which love, we do fulfill God's Law, the same Law Christ commanded of us to fulfill. And as to the rest of 'the Law', Christ took care of that for us....
So, through faith, we do not void law, we establish law. That is, faith is not the instrument which is used (through) to void law, but is the instrument which is used (through) to establish law.
Let us all say, 'Lead on Spirit'.