To fish for Offin in Mario & Luigi: Brothership, you need to follow these steps:

1. Connect Lottacoins Island: The fishing mechanic is unlocked after connecting Lottacoins Island, which prompts a Concordian message that allows you to engage in fishing activities.

2. Access the Fishing Spot: Once the fishing mechanic is unlocked, you need to find the specific location where you can fish for Offin. Unfortunately, the exact location for Offin is not specified.

3. Check for Q&A: Look for questions related to fishing and specific fish like Offin. This might provide more specific information about where to find Offin.

4. Watch Gameplay Walkthroughs: Watching gameplay walkthroughs or videos specifically focused on fishing in Mario & Luigi: Brothership might help you identify the location where you can fish for Offin.

5. Consult General Guides: General guidance on fishing mechanics and locations might offer some insights, although it may not specifically mention Offin.

By following these steps, you should be able to locate the spot where you can fish for Offin in Mario & Luigi: Brothership. If you still encounter difficulties, consider checking more recent posts or updates or watching additional gameplay videos for more detailed information.