They are supposed to approve of each candidate individually. This avoids problems such as the one you allude to in your example where Sanders may lose to Clinto

They are supposed to approve of each candidate individually. This avoids problems such as the one you allude to in your example where Sanders may lose to Clinton despite more total people approving of Sanders, simply because Clinton supporters (the larger group) rank Clinton>Sanders while Sanders supporters (a minority of voters) only rank Sanders.