This is a step-by-step guide on how to install custom ROMS on the Galaxy S5. You can install a wide variety of ROMS ranging from AOSP ROMS like CyanogenMod and Paranoid Android all the way to lag free Touchwiz custom ROMS.
Note: This will remove everything from your internal storage backup anything and everything! Also I highly recommend you always perform a Nandroid Backup before you install a custom ROM!
You must have a rooted Galaxy S5
T-Mobile/Canadian users! You can also use the international SM-G900F Galaxy S5 variants ROM too!
Places to download custom ROMS for the Galaxy S5. Just select “Android Development” or “Original Android Development” below.
Samsung Galaxy S5 International (Europe, Asia, etc)
Samsung Galaxy S5 T-Mobile/Canadian
Samsung Galaxy S5 Sprint
Remember to always read the developer’s directions before flashing a custom ROM.
1. Download a custom ROM directly to your phone or to your computer and then transfer it to your phone. I’m going to use “Allusion” ROM as my example.
2. Enter your phone in Recovery Mode. Power off your phone. Press and hold the volume up, home button, and power button.
3. Select “Wipe data/factory reset” and then “Yes”. NOTE: This will delete information off your phone including apps, call logs, settings, messages, etc.
4. Then choose “Wipe cache partition”.
5. After that go into “Advanced”.
6. Then select “Wipe dalvik cache”. After that select “+++++Go Back+++++”.
7. Choose “Install zip from sdcard” > “choose zip from sdcard”.
8. Locate the ROM you download/transferred on your phone in step 1 and select it then choose “Yes”.
9. Once complete choose “+++++Go Back+++++” > “Reboot now”.
Wait for about 5 minutes for your phone to reboot and you are now on a custom ROM! Enjoy flashing TONS of ROMS, BUT BE CAREFUL!
FAQ:
Q: After flashing a custom ROM I’m stuck on the boot loop, what do I do?
A: Preform steps 3-6 again.
Q: What is Gapps?
A: Gapps stands for Google Apps, some ROMS require you to install this package after the custom ROM to get Gmail, Play Store, Hangout, ect. Mainly all AOSP (pure Android) based ROMS require you install this after the ROM.
source: http://www.mediatekroms.com/2015/07/how-to-install-custom-rom-on-samsung-galaxy-s5.html

Why would I need a custom rom in Samsung Galaxy s5