It doesn't start with making them study "on their own" because that would only give you the opposite results. Parents should understand where this lack of interest is coming from, and that can be done by talking to them. Do not scold them for their reasons but try to recognize and acknowledge what they've been going through. There are times when a child isn't motivated to study not because he or she doesn't want to but because of some reasons that are outside the academic sphere.
However, if you're child is really struggling with his or her lessons, and doesn't want to study because they think they wouldn't understand it anyway, then as a parent, you need to be their go-to person. You need to be that person whom they can tell these problems to without getting the feeling that you won't be proud of them anymore. I think telling them that you have gone through the same troubles when you were young, but eventually overcome such failures will give them the feeling that if their parents can do it, they can do it as well. Make them realize how studying has helped you in the most relatable way as possible for them to realize how important it is to feed their mind. Talk to them about their goals in life and how studying can help them achieve such goals. I also think it would be helpful if you will join them in studying until the time they get used to it and would choose to do it on their own.