I'm trying to play Xbox Live from my Xbox 360. I've done a Game Pass, logged into Xbox with that profile but then, I'm lost. I need a step by step guide to make this work to play online.
According to Microsoft FAQs, Game Pass subscriptions are not available for Xbox 360:
For a low monthly price, you can enjoy access to more than a hundred great Xbox games—including some games made for Xbox 360 and the original Xbox. Game Pass subscriptions are available for use on Xbox Series X|S, Xbox One, PC, and your mobile device. Xbox Game Pass Ultimate works on console, mobile device, and PC, and it also comes with an Xbox Live Gold subscription.
Since you are using an Xbox 360, it won't work by the sounds of it.
Second, do you have a Game Pass Ultimate subscription? Only Game Pass Ultimate includes an Xbox Live Gold subscription, which is needed for multiplayer services (except for free-to-play online games), according to the FAQs:
Can I play multiplayer games with Game Pass?
Depending on the game, yes—though for multiplayer you need either Xbox Game Pass Ultimate or a subscription to Xbox Live Gold (sold separately).
Note Free-to-play online multiplayer games no longer require an Xbox Live Gold subscription.