You need to have read the whole thing, over and over and over, until it finally starts to sink in. You need to study it with other Christians so you get your ideas straight. That's why belonging to a small group is so important. Those who are mature as Christians are generally those who have read and studied God's word for years and years, who are constantly applying it to their lives, as a corrective, as a guide. Again, every part is needed.
Every part needs to be working properly for the body to be built up.
By growing up. By becoming more like Christ as we grow into him. So speak the truth in love, and grow into Christ who is our head, Do your part in bringing the church to maturity, to unity, to the full stature of Christ.