This is quite complex but rather fun once you settle down to it.
Lots of cultures think 'star signs' are important. So why not those in a fantasy world or even a far future one? With, of course, quite different stars in their skies from the ones that wheel overhead in the real world. So, no Libras or Leos or whatever (and no, I don't know mine so don't ask!).
Settle down with these tables and come up with some authentic-sounding astrological patter that works for YOUR world, wherever and whenever that is...
To start with, our star sign needs a name. Roll a 1d6 to find out the sort of constellation name we are looking for... OK, it is a 'Name the Profession' one (this will make sense in a minute, promise).
Get a 1d2 to determine which combination of name tables you'll roll a d20 on... that gives me Lilerta... um... next, I need to trundle off to the Professions table, and roll another d20... um... a Shepherd.
OK, Lilerta the Shepherd it is. Now to find out which season and where in the sky to find it... South sky in summer... and finally the usual astrological generalities, a good and a bad feature common to everyone born under that star sign... us Lilertans are Wise but Passive.
Maybe a bit long-winded, but the results are worth it for that added realism to round out your alternate reality.