Two parties are needed to make a new heart. The "new man" is the New Covenant man. He is the man to whom God has given a new heart and in whom He has placed a new spirit (Ezekiel 36:26). Here, God takes the initiative; it is His doing.
Yet, notice the change in terminology in Ezekiel 18:31: The responsibility becomes ours! "Cast away from you all the transgressions which you have committed, and get yourselves [make you, KJV] a new heart and a new spirit." In this passage, it is man, not God, who creates the new heart and the new spirit. Ezekiel 18 and Ezekiel 36 do not contradict; by Himself, God cannot create the new man in us. He needs our cooperation.
The Hebrew word translated "make" in Ezekiel 18:31 (KJV) is asah. God uses it some 2,625 times in the Old Testament. The translators render it a number of ways.
» To make in the sense of fabricate or build: "God made the firmament" (Genesis 1:7); "And you shall make holy garments for Aaron" (Exodus 28:2); "I did not make an end of them [the children of Israel] in the wilderness" (Ezekiel 20:17). Asah does not imply creation out of nothing - the Hebrew word bara, used only 60 times in the Old Testament, carries that meaning: "In the beginning God created" (Genesis 1:1). God is always the subject of bara, but as we can see from the examples, He is not always the subject of asah.
» To execute in the sense of "to do": "[Y]ou have established equity, You have executed justice and righteousness in Jacob" (Psalms 99:4). "Remove violence and plundering, execute justice and righteousness" (Ezekiel 45:9).
» To prepare, especially a sacrifice: "And when you prepare a young bull as a burnt offering, or as a sacrifice to fulfill a vow . . ." (Numbers 15:8; see also verses 5, 6, 12). "For since the beginning of the world, men have not heard, . . . what He ha[s] prepared for him that wait[s] for [H]im" (Isaiah 64:4, KJV).
» To work: "He has filled them with skill to do [KJV work] all manner of work of the engraver" (Exodus 35:35); "Then Jonathan said, . . . '[I]t may be that the LORD will work for us" (I Samuel 14:6); "So we built the wall; . . . for the people had a mind to work" (Nehemiah 4:6).
» To commit: "But if a wicked man turns from all his sins which he has committed, keeps all My statutes, and does what is lawful and right, he shall surely live, he shall not die" (Ezekiel 18:21).
» To do: "And I gave them My statutes, and showed them My judgments, which, if a man does, he shall live by them" (Ezekiel 20:11). "I am the LORD your God: walk in My statutes, and keep My judgments, and do them" (Ezekiel 20:19).
The conclusion is inescapable: asah, translated "make" in Ezekiel 18:31 (KJV), is synonymous with keep, do, work, and similar verbs. We "make [ourselves] a new heart" by what we do! Specifically, the action God requires of us is keeping His law, doing His commandments. This is a Christian's work.
By its meaning of "prepare," asah describes both sides of the covenant agreement. It describes what God does for us and what we must do for ourselves if we are to receive the promises of the New Covenant.
God, for His part, has prepared unimaginable glory for us, as Isaiah 64:4 makes plain in the KJV (see I Corinthians 2:9). We are to prepare ourselves just as an Israelite prepared an animal sacrifice (see Numbers 15). It is up to us, as "living sacrifices" (Romans 12:1), to prepare ourselves for the marriage of the Lamb by putting on clean clothes - the new man (compare Revelation 19:7-9 with Colossians 3:9-10).