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What's complicated about it? You run a client ("i2p router") and configure your browser's proxy as 127.0.0.1.

And then what?

And then you can browse .i2p websites.

Where do you get them? Is there some kind of catalog? (don't answer if it's not supposed to be in normal internet)

Haven't tried it, but the primary i2p software comes with an address book according to their website.