: Immediately after the ROM is finished, repeat the "Install" process for the GApps ZIP.
If you are looking for Android 5.x (Lollipop) or higher, you must install a Custom ROM (like LineageOS), as Samsung stopped official updates after Jelly Bean. gti9100 firmware 511 download upd
Specifically the Pico version, as larger packages may fail due to the S2's limited system partition. : Immediately after the ROM is finished, repeat
These sites allow you to search by CSC (Country Specific Code). If you know your original region (e.g., XEU for United Kingdom, XEF for France), you can find the exact "511" variant you need. XEU for United Kingdom