With the nock attached to the arrow, I hold the nock in a pan of boiling water for 15 seconds or so, long enough to soften the plastic a little. Then I press the nock onto a piece of copper wire I have that is the same fit I want on my bowstring. You should be able to find something, a nail, etc., that will work for you. It the nock is too loose, you can pinch the nock ends together after holding it in boiling water. The nock will hold its shape after it cools. If you try to enlarge a nock or make the opening smaller when the nock is cold, it can fracture the plastic and one of the tips of the nock might break off the next time you shoot the arrow, causing a dry-fire.