She believes that a thorough knowledge of your topic and plenty of practice is leading to success.
Explanation:
The first verb to believe is used in present simple tense because it refers to her habits or general truth. In this example, she expresses something in what is she certain. The second verb to lead is used in present continuous tense because this action will be also present in the future and refers to a permanent situation that will continue and does not stop in the present moment.
