Download MIUI 1.8.12 here
What do you need before flashing this ROM?
1. You must be in the Gingerbread version (2.3)
2. Have recovery mode installed (Home+Volume UP+Power)
If you don't have this I recommend you install DarkyROM since it's Gingerbread, rooted and have CWM pre-installed.
How to install DarkyROM, click here
Note: All you apps will be wipe-out
Once you have Gingerbread in your phone, rooted and CWM available. You may flash to MIUI now.
1. Copy "MIUI.us_galaxys_1.8.12_Eng_Deo_ZipA_Signed.zip" to your "/mnt/sdcard" folder
2. Reboot recovery (HOME+Volume UP+Power button)
3. Select ZIP file to install (optional to clear cache and dalvik. Do it if you like fresh)
4. Sit back and relax...
Continue reading click here