Properly licensing a plugin that uses Apache 2.0 licensed code

From WordPress point of view — not much, beyond plugin’s header (and even there) it is hardly policed.

From licensing point of view you obviously need to accommodate requirements of both licenses accordingly. At least — make clear that your combined work is under GPLv3 and which parts of it are based on Apache License 2.0 code.

GPL site has instructions on how to apply terms, although recommendation to have it done in every source file is widely disregarded in practice.