The text reads ‘speaking to yourselves’, and it is ambiguous in the English translation. It could be understood to mean, speaking to oneself, and you can certainly speak to yourself in Scripture in hymns, psalms, spiritual songs, and think to yourself, and encourage yourself, but the Greek makes clear that it is speaking to one another.
There are rules for mutual encouragement and teaching. Do you know the person well enough? Suppose it is an admonition. You see a brother or sister in Christ doing something which is injurious and wrong. Are you a friend? Can you come alongside and pray for grace, and so help your fellow believer? Are you willing to receive it from others? If you are and you have the humility to receive admonition from fellow Christians, then maybe you are qualified to also give it, but if you are a haughty person and not willing to receive it, don't ever give it. You have to be willing to confess your own fault. You may be the perfect person to admonish another, because you made that very same mistake and down you went and it had serious consequences for you. Well then you are the ideal person because you can say, ‘I did that too and this is what I found, and this is how the Lord dealt with me’, and that will be received, because there is a humble approach. Remember that if a person goes astray in our spiritual family, it may be our fault, because we saw it and we never said anything; we never got alongside.
How do I make melody in my heart? It is not about trying to reproduce a tune in your imagination. You can do that, but it is not about that. It is about allowing the terms that you sing to move you so that you mean them. It is about deep sincerity and being touched by the sentiments of the hymn, so that you are truly grateful to the Lord, truly sorrowful for your sin, truly astonished and moved at the love that ever brought him to Calvary to suffer and to die.
What shall we do with the Psalms? We can read them. They are great songs of spiritual experience so richly worked out. But then we can sing them. In what form? Shall we sing the Scottish paraphrases, which stick very closely to the precise words of the Hebrew? Well, some of them are very full of meaning and very beautiful, just as they are. But this presents a problem to us, because they use the language and the theology of the Old Testament. They sing about sacrifices and about all the Old Testament order, and use Old Testament language. Other than obliquely in words of prophecy, they don't explicitly mention Christ, because he had not yet come. What we need to do is Christianise them and translate the Old Testament symbols and language into its New Testament fulfilment. In England, many people did it, and perhaps the best-known name for doing it was that of Isaac Watts. Without this, you would never sing the name of Christ; you would never sing about the blood and conversion, or the return of Christ. We can prove this to ourselves by looking at Revelation 4 and 5 and asking ourselves where all the elements of New Testament worship that we see there are found in the psalms?