INSTALL

apt-get install quota

CONFIG

Add options usrquota,grpquota in /etc/fstab:

UUID=a5bcc8d7-asdf    /    ext4    errors=remount-ro,usrquota,grpquota 0   1

Set quota:
setquota -u -F vfsv0 USERNAME 4000000 5000000 0 0 /
4 000 000 ≈ 4GB.
OBS: block size is always 1 KB as unit, regardless file systems.
Alternatively, edquota USERNAME can be used which brings the nano editor.

REBOOT

reboot

CHECK

repquota -a #report all users
quotaon -pa # check if on