To be able to install a poster to your device, you need to disable Find My. This can be found in Settings > iCloud Account > Find My > Find My (iPhone / iPad)
Download the app for your operating system at this link.
Install either Apple Devices (from Microsoft Store) app or iTunes (from Apple website).
Use a USB-C or lightning cable to connect your Apple Device to your computer.
Click the refresh button in the top left corner of Nugget. The dropdown to the left should now have the name of your device inside.
(Troubleshooting: Try a different cable, Ensure Apple Devices or iTunes is installed and running)
Select the Posterboard tab on the left of Nugget. Next, enable the Modify checkbox.
Using the button labeled "Import Files (.tendies)", import whichever Posters you would like to install
On the left, select Apply. Then "Apply Changes". Your device will now restart.
After your device restarts, navigate to the wallpapers screen. You can find your new wallpaper under the section titled "Collections". If you do not see your new wallpaper, do not worry as it can take a few minutes for it to show up.
You are now able to re-enable Find My on your Apple Device.