Some say stand on you head, spin three times and call yourself nancy!.
Some say it doesn’t matter as long as you don’t go into the dungeons.
Some say don’t talk to Siegeard at all.
Some say talk to Siegward first, then send greirat.
Some articles say to send Greirat once you get to the central bonfire.
After you beat the boss, Greirat should be back with some new weapons for sale. You’ll want to send Greirat to pillage and then you should run across Onionbro when you’re going through the area. You need to save Onionbro from the well outside the Cathedral bonfire. Should I send Greirat to pillage Irithyll? Not likely, I killed Patches when I first met him in Cathedral of the Deep and Greirat still died in Lothric Castle. Killing him (Patches) removes the chance of him being betrayed and killed and thus he survives.