Installing Drain in tar + gravel roof ??

Hello. My tar + gravel roof has only 3 drains, i.e. one corner has no drain. I want to install one. An advice appreciated re: removing material, cutting hole for drain, and what to use to replace removed material .. membrane ? .. what to use at border of membrane + existing roof material ?? Roof patch compound ??
Please reply to ng. Thanks. -JS
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