Westminster Project 1.6

6. The whole counsel of God, concerning all things necessary for his own glory and for mankind’s salvation, faith and life, is either expressly written down in Scripture or may be deduced from Scripture by good and necessary consequences. Nothing at any time is to be added to Scripture, whether by new revelations of the Spirit or the traditions of mankind. Nevertheless, we acknowledge the inward illumination of the Spirit of God to be necessary for the saving understanding of such things as are revealed in the Word, and we acknowledge that there are some circumstances concerning the worship of God and government of the Church, common to human actions and societies, that are to be ordered by the light of nature and Christian prudence, according to the general rules of the Word, which are always to be observed. 
(WCF 1.6)

One of the first things they teach you when becoming a pilot is: “Trust your instruments!” This was told to me by an extremely well-traveled friend who has flown planes to every part of the globe. That phrase, “trust your instruments,” has stuck with me. Why? Because he said that no matter what you feel or how much experience you think you have, when you’re up there in the air, your instruments can be the difference between landing safely on the tarmac or crashing into the ocean floor.
I mention this because that’s what the Westminster Confession wants us to see here: the Christian life is a journey, and God has given instruments to His church so that we might sojourn safely. The instruments (at least those discussed here) are the Bible and the Holy Spirit (2 Peter 1:20-21; John 14:15-31, 15:26-27). Understanding God by and through His “special revelation” is the difference between spiritual blessedness and total ruin (Psalm 119).
You see, the Confession wasn’t written in a vacuum. Historically, on one hand, the church was pushing back against Roman Catholic traditions that hadn’t yet been “weeded out” of the Church of England. On the other hand, it was opposing a movement known as the Anabaptists, whose many and varied teachers claimed to have had “private revelations” that were on par with—or even trumped—Scripture. (Of course, they all contradicted one another; cf. Galatians 1:6-9, 2 Thessalonians 2:2.)
The Confession says no. Want to know what is sufficient for the church’s life and doctrine? It’s found in what God has given so that we might be “complete and equipped for every good work” (2 Timothy 3:17, ESV). In other words, the church is called to trust in the “instruments” God has given us because He has provided everything we need to be saved and to lead a life of faith pleasing to Him (John 6:45; 1 Corinthians 11:13-14; 14:26, 40).
However, the Confession acknowledges that the Bible doesn’t always specify every “what and how” regarding ecclesiastical practices. Thus, the church sometimes must do the hard work of deducing what God would have us do—for example, how to administer baptism or the appropriate age for taking the Lord’s Supper. But this is where the Confession brings us back to the foundation: even if it’s not always clear, we reason and work out what to practice by and from the 66 books of our God-breathed Scriptures.
A couple of things to note as we reflect on this. First, the Confession rejects “new revelations” from the Holy Spirit, but in no way does it reject the Spirit’s very personal and ongoing work in the lives of His people. The third person of the Trinity is vitally important—it is He who “illuminates” the truths that have always been in Scripture so that we might apply them to our context (1 Corinthians 2:9-12; Ephesians 1:17-18). He leads and guides us into all truth (John 16:13).
Second, just as the Confession wasn’t written in a vacuum, neither do we live in one. Because of God’s good ordering in creation, the church can take note of and apply truths found in “common human actions and societies.” We are to worship God—do we meet for two hours or five? Once or twice on Sunday? In a building or in homes? We are to have elders—do we have two or ten? We are to preach the Word—what translation do we use? How long should a sermon be?
The Bible is silent on many of these questions. What matters is that “the general rules of the Word are always to be observed”—meaning that which God has made clear we must follow. For the rest, we must seek His wisdom so that, together as His people, we can work it out in love and Christian unity.