The best way to practice copyediting is to find a piece of writing that you really like, and then edit it yourself. Try to find errors in grammar, punctuation, and spelling. Also look for awkward phrasing and sentences that could be worded more clearly. As you edit, keep a style sheet next to you so that you can keep track of the changes you make.
If you don't have a style sheet, you can also use a word processing program to track changes. Most word processors have a "track changes" feature that allows you to see the changes you make to a document. This can be a great way to see your progress as a copy editor.
If you want to get more practice, there are also many online resources that provide editing exercises. Grammarly is a great website that offers editing exercises and quizzes. The Purdue Online Writing Lab also has a section on copyediting that includes tips and exercises.
