Q: How doth God execute his decrees?
A: God executeth his decrees in the works of creation and providence.

Revelation 4:11. Thou hast created all things, and for thy pleasure they are and were created.
Ephesians 1:11. In whom also we have obtained an inheritance, being predestinated according to the purpose of him who worketh all things after the counsel of his own will.

Westminster Project - For more info, follow the Link to Matthew Henry's commentary on the WSC.