The best time to do this is in the off season so that your brain and body have time to learn the new action.
1. Begin by standing still and shadow bowl as if bowling at the batter in the reflection.
Check your arm action, is it straight? If not, then self correct and straighten it as it comes over.
2. As you become proficient at this, take a couple of steps back and walk through your action, monitoring your bowling action, making sure that your bowling arm stays straight throughout the release.
3. As you improve, probably after a couple of weeks, you can progress by jogging toward the mirror or window reflection and shadow bowl through your action.
4. Keep progressing this as your action becomes stable and you can repeat your action without it regressing into a throw.