Yup, if memory serves, you want to set up the account, friend your *main* account (one sends request, other adds to friend list, so you're both friends when you fire up the game on your birthday account), and then start up GT5. Once you go through all the first-time setup stuff (create your driver, set your desktop appearance, etc.), you get the birthday card ticket. I believe it's at that point that you quit the game and save your data to a flash drive. Go back into the game and open the ticket to see what you got; if it sucks, quit, delete that save file (or just write over it) and copy the one on your USB stick. Open the game again, see what you get, etc.
And when you get something really choice, you send it to your other (main) account. Done.
Or you could delete the save once you've sent it, and reload from the USB, and keep trying 'til you get a SECOND really cool car. You can't send that one, though, so you have to stop there, unless you have more USB drives that you can save onto.
Meanwhile, do a Google search of "GT5 birthday cars" and you should be able to find a list of what cars come your way in what years. Apparently it's random, but from what I hear the really nice ones don't tend to show up very often. But I know one guy who did '92 as his "birth year", and managed to collect the whole list of '92 cars:
Mercedes-Benz 190 E 2.5 16 Evolution II Touring Car 92
Mazda éfini RX-7 Type RZ (FD) 92
Lancia DELTA HF Integrale Rally Car 92
Mazda RX-7 Type RZ (FD, J) 92
Honda CR-X del Sol SiR 92
Honda CIVIC SiR-II (EG) 92
Nissan R92CP Race Car 92
Peugeot 905 Race Car 92
Honda BEAT Version F 92
Mitsubishi Lancer Evolution GSR 92
Honda NSX Type R 92
Mazda Autozam AZ-1 92
Ferrari F40 92
Jaguar XJ220 92
Toyota SERA 92
Said it took him a few hours of doing it and redoing it - this was in the days when you could send cars all day long, so every time he got something he didn't have already, or something even vaguely cool, he just sent it. He said he was after the Peugeot 905, and by the time he got it, he had THREE R92s, FOUR F40s, and had gotten the others numerous times.
I know the Toyota 7 is on the list for 1970, but I don't know anyone who's gotten it via the birthday glitch.
'67, '70, '92, '89 all seem to be pretty good years to try, with maybe more good cars than bad in those years.