What are the steps to cutting polyurethane?
I normally would not recommend thinning urethane. If you have a new can of fresh urethane, it should be ready to go. I would try to apply it thin rather than a thicker coat. If you feel you really need to thin it, just slowly pour some in another container and add a small amount of thinner at a time. Stir it slowly and you will have no issues with air bubbles. Never shake or use a high speed mechanical mixer.  If you see some small bubbles, don't worry. They should dissipate during application. the only time air bubbles are an issue is when the product is shaken hard and the urethane appears real cloudy.