1. When you go from 1-1/4" drain pipe to 1-1/2" you need a 1-1/4"x1-1/2" slip joint washer.
2. It's advisable to have all vertical drains in plumb position. There are off set connectors that you can use if they don't align.
3. The leak can also originate at the drain set (top of the green - bottom of the sink).
4. Can't do it? get a plumber to help you. You'll save time, money, aggravation and cussing.