Step 1: Request a free Data-SIM, you can find it under your account, manage your plan
fi.google.com/data and enter the 16 character key from the SIM Card
Step 3: Insert new SIM Card and enjoy.
* You can rename the data card within your Project Fi account so you can easily monitor all of the additional data (ie. Data for iPad, Data for old Phone, etc.)
* Depending on the device you installed the additional SIM card, you may have to go to device setting to enable 3G or LTE or auto-detect; most of the non-Nexus devices may not be able to maximize the best bandwidth between T-Mobile or Sprint (in most GSM phones, it defaults to T-Mobile and depending on the bands available, you may be stuck with slower network connections like H+ or HSDPA), a good tool is LTE Discovery App to see what band you end up with.