Check the Cyanogenmod -Wiki:

• Download: http://get.cm/?device=jfltexx
• Howtos:
  • http://wiki.cyanogenmod.org/w/Install_CM_for_jfltexx
  • http://www.droidviews.com/how-to-install-custom-roms-on-samsung-galaxy-s4-gt-i9500/

• Android 4.4 uses SElinux in “enforcing” mode: installation recovery via CWM recovery app impossible
• Use flashify app to install downloaded recovery image

• Backup!
  • Nandroid (needs ~2gb on sdcard) +
  • Titanium backup +
  • Rsync to linux via ssh server +
  • Backup /efs/ folder - it contains IMEI and phone data
• Heimdall install and build fails on debian wheezy (libusb probs), but works in Ubuntu x386
• Use Clockworkmod ROM- Manager to boot into CWM recovery, if it doesn't work by booting with buttons HOME + VOL-UP + Power pressed
• Backup from CWM recovery via “Backup to external SD-card”
• Wipe before install:
  • Fresh install coming from another ROM → Fullwipe: “wipe data / factory reset”
  • Cyanogenmod Update → “wipe cache partition”
• Take care to install the correct version (unbranded / branded / US / Intl / LTE / …)
• Troubleshoot: http://www.droidviews.com/things-to-do-before-and-after-installing-a-rom-troubleshooter/

• Bootloop: Try to “wipe cache” and “wipe data / factory reset”
  • http://www.droidviews.com/how-to-unbrick-your-bricked-samsung-galaxy-s4-gt-i9500-gt-i9505/
• Install Official Firmware - Howtos for Samsung S4
  • http://www.droidviews.com/how-to-install-latest-official-firmware-on-samsung-galaxy-s4-all-models/
  • http://samsung-updates.com/device/?id=GT-I9505