Yeah, that would be roll-your-own, if you're going to be contacting the advertisers yourself.
I'm not an expert in this area (I've used Adsense a little, but not much more than that). The basic idea is that you have to mod your templates to put in your advertising banners in the appropriate places.
There are a couple of mods that either help you with this (SubTemplates) or that help manage your advertising overall - rotating ads, etc. If you search over on the phpbb site, you'll find a lot of lively discussion on the topic!
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