Topology:
Linux-based PC: 192.168.1.1
Linux-based Laptop: 192.168.1.7

In PC, you edit /etc/ssh/sshd_config using any editor you like:

$ sudo vi /etc/ssh/sshd

Looking for this line, or you can add if it's not already there:

X11Forwarding     yes

Save that file and restart SSH service:

$ sudo /etc/init.d/ssh restart

Allow Laptop to run X applications:

$ xhost 192.168.1.7

Define default display:

$ export DISPLAY=192.168.1.1:0

Then from Laptop terminal, run the command to execute any X applications you like:

$ ssh -X [username]@192.168.1.1 firefox

$ ssh -X [username]@192.168.1.1 oowriter

That's it! :)