Needs to be done before beating the Abyss Watchers. I recommend beat the Deacons before doing this. Give the old lady Dreamcatcher's Ashes and talk to Sirris so that she becomes friendly with you.
- After offering Rosaria a tongue, do not go back to shrine and keep moving to Keep Ruins. Summon yellow finger near the house full of slugs, and get his gesture.
- Beat the Abyss Watchers, move on to Irithyll, and kill the gator.
- Rest at bonfire and go back on the bridge, use the summon sign to help Sirris beat that guy and keep going on Irithyll until you are invaded by him after the next bonfire (Yorshka Church). Kill him and go back to the bridge and get his loot.
- Do not go back to shrine before defeating his invasion.
- Go back to the Shrine, talk to Sirris then she will become hostile to you. If you want you can kill her at this point to get her armor.
Normally if you decided to complete Sirris' questline you cannot get yellow finger's gesture, but this way you can get the gesture AND dragonslayer's axe. But not her ring and Hodrick's armor. I missed both the gesture and the axe on my first playthrough so I did what I described and got both of them on NG+.
The key point here is, if you don't go back to the shrine and talk to Sirris after offering a tongue, it does not trigger her hatred on you.
I haven't tested if only going back to the shrine and not talking to Sirris would do the same yet.