1. Check there is enough space in /home folder. Because the customized OS will be stored under /home folder by default.
2. sudo /etc/apt/sources.list
Add this line at the bottom of the file
deb http://www.remastersys.com/ubuntu precise main
3. sudo apt-get update
4. sudo apt-get install remastersys*
5. sudo remastersys dist iso mycustomubuntu.iso
6. Goto the location /home/remastersys/remastersys
7. md5sum /home/remastersys/remastersys/yourfilename.iso
8. Check the file that is located /home/remastersys/remastersys/youfilename.iso.md5
Both the output should be the same.
9. sudo remastersys clean
