If you killed the woman at the Riften orphanage, as per the quest for Aventus Arentino, then you usually have to wait approximately 3 days for the letter to arrive.
If it's been more than three days, try standing in a secure location where a courier can reach you, like a Hold Capitol, and wait (not the select version, literally waiting, go make a sandwich, surf the internet, give him time to reach you).
Don't go too far from the game, as this is one of the few courier deliveries that has dialogue options, though I think you receive the Mysterious Note before the dialogue options.
If you wait for more than the time it take to make and enjoy your sandwich, and you still don't see the courier, check your inventory under books to make sure you didn't just miss the courier's delivery
Again, you're looking for the "Mysterious Note". If you have it, just sleep in a secure location (preferably an inn). If not, try sleeping at an inn anyways.
If it doesn't happen, congratulations! Either the Brptherhood doesn't want you or, more likely, your game is glitched, and chances are the only way to fix it is to nix this game and start a new one. Sorry if I didn't help, I know you said you've done this questline before, but sometimes people just forget the steps.