Your question is really about how to develop any body paragraph in an essay and how to transition from one to another. This question is really easy if you remember one thing--explain, use examples and prove from the text. If you are discussing the disadvantages of sports and one of them is injuries, don't simply list injuries for various sports. Take one sport you are familiar with and show the reader what happens when a player is injured. If you use hockey or soccer, you could discuss the long term effects of concussion, the possibilities of being paralyzed, the knees or ankles needing surgery to correct injuries etc. The idea is to SHOW, not tell with a simple list. Put a picture in your reader's mind. As for transitions, you need to help your reader follow your thoughts and the order in which you present material using a connection between them. If you are talking about concussions and their consequences for a player, you cannot simply switch to surgery without a connection like this: "Another disadvantage of sports injuries is the possibility of surgery to correct a problem such as a high ankle sprain which is worse than a break." I hope this helps.