It's not that hard, fire up the GT5, go to Community, press Remote Race than press Stand By mode. leave the PS3. Fire up the PC, go to granturismo official site, go to GT5 (my) home and login with your playstation id and password. Once you loged in, you should see a bar with 'ready to host remore races', press that one, a new page should be loaded. Select your own driver that you would like to be raced, than select the other drivers you would like to be raced. press 'start race' and you're off.
You could also use the B-Spec grinder...
Hope it helped!
Greets!
PS: I use the Belgian site...I simply translated all names so it could be a little diffrent.