Galaxy Note 3

Enable Developer options

In Android 4.2 and later, developer options are disabled by default. To enable them, go to About Phone, and tap Build Version seven times. Now you can enable things like

USB debugging (for use with the Android SDK),
an advanced reboot menu, and
hard-killing apps with a back button long-press.

Installing the CACert root certificate

The root certificate:

wget https://www.cacert.org/certs/root.crt
cat root.crt > 5ed36f99.0
openssl x509 -inform PEM -text -in root.crt -out /dev/null >> 5ed36f99.0

Rinse-and-repeat for the Class 3 certificate:

wget https://www.cacert.org/certs/class3.crt
cat class3.crt > e5662767.0
openssl x509 -inform PEM -text -in class3.crt -out /dev/null >> e5662767.0

Copy these two files onto your phone, then on your phone using the Terminal app:

su
mount -o remount,rw /system
cp /sdcard/5ed36f99.0 /sdcard/e5662767.0 /system/etc/security/cacerts/
cd /system/etc/security/cacerts/
chown root:root 5ed36f99.0 e5662767.0
chmod 644 5ed36f99.0 e5662767.0

Then reboot your phone.

Or use the Android SDK

If you can't be arsed typing all that into a terminal on your phone, fair enough, it's quite tedious. Instead, install the Android SDK, and:

~/android-sdk-linux/platform-tools/adb push e5662767.0 /sdcard/
~/android-sdk-linux/platform-tools/adb push 5ed36f99.0 /sdcard/
~/android-sdk-linux/platform-tools/adb shell

Then type it here instead. You'll need to enable USB debugging first, which is a developer feature (See above.)