legacies

how do you even let a sorority know you're a legacy?

It depends. Sometimes it guarantees you to just the second round sometimes to the 3rd. Different policies for different chapters. It is a good thing to be a legacy but not just to "get to the next round" but also because if it is your mom or sister or grandmom that makes you a legacy, that is a wonderful connection to be able to share with them! I am a legacy with my mother and sister to my chapter and it was so meaningful for me to join and get initiated to the same chapter as them.
So whoever you are the legacy by (AKA your mom, sister or whoever it is...) can log on to her national sorority website (have her call their HQ if). She should download the official legacy introduction/recommendation form, which she can fill out for you. Then she should send it to her chapter at UD ASAP (only because the chapters will be inundated with recruitment stuff come February so it is best to get it in sooner for their sanity's sake :) ). There may be 2 forms depending on the sorority, but if they call the sorority's HQ they should be able to help out.

What if I am a legacy but the person who was in the sorority is deceased?

In that case if your legacy relative is deceased, contact the national headquarters of her sorority to find out if you can submit this form on your own, or if another member of the sorority may do it. Nationals of chapters want to recruit legacies usually so there should be a way to register you as a legacy still.

ALSO remember, different chapters have different legacy statuses. For some they consider sisters, daughters, granddaughters, great granddaughters, aunts, step siblings/daughters/granddaughters....
Then some only consider immediate family like daughters and sisters.
So make sure you know what chapters consider what a legacy.