The people whom I formed for myself describes God’s people. The verb formed is an inclusio for verses 1-21 since it occurs here and verse 1 (see the comments there). The phrase for myself expresses God’s ownership of the people of Israel and his close relationship with them.

That they might declare my praise: God made a covenant relationship with the Israelites, so that they would praise him. Declare my praise indicates that their praise of God must be public, so that others know what he has done. They are to be witnesses for God (see verses 10, 12). Good News Translation has “sing my praises,” and New Jerusalem Bible says “broadcast my praises.” New International Version and Revised English Bible are similar with “proclaim my praise[s].”

For the translation of this verse consider the following examples:

• … those whom I formed to belong to me,
so that they would praise me and make others praise me.

• Those are the people I created to be mine,
so that they would praise me publicly.

Quoted with permission from Ogden, Graham S. and Sterk, Jan. A Handbook on Isaiah. (UBS Helps for Translators). New York: UBS, 2011. For this and other handbooks for translators see here .