The catch with using Fi is you need a Fi phone to activate your plan. So if you’re running with the Gemini, you’ll need to borrow a Fi phone from someone to activate your plan. A quick google search will give you pointers. I had already had a Fi phone — I chose Fi back when their data plans were a ridiculously good deal and I needed international roaming for data. They’re still a pretty good deal, and they don’t charge extra for data SIMs, which is why I stick with Fi.
Anyway, once you have a Fi data SIM, it’s something like this to set your APN:
From the home screen, tap the Menu button.
Tap Settings
Tap Mobile networks
Tap Access Point Names
Tap the Menu button
Tap New APN
Tap the Name field
Enter “h2g2”
Tap Save
Technically you don’t need to set the APN, but you won’t have international data without it, as I discovered at one point. It *should* just work when you slot the SIM by connecting to T-Mobile; Fi is just an MVNO on T-Mobile.
I suppose at this point I should offer the obligatory disclaimer. I work for Google, not for Project Fi, and this is known but unsupported, blah blah blah. If it’s any comfort, I did this BEFORE I took the job from them about a year and a half ago, with an iPhone.