This may be a very simplistic guide, but it will be an Usefull one I promise you.
1- Reach the Final part to Seigward´s quest
https://www.youtube.com/watch?v=YFPSaTl5nOg&t=338s
(This is an Link to a video going through each step of how, and where to encounter this Sweet NPC and how to progress his questline)

2- Do not pick up the STORM RULLER

Yhorm being a gigantic powerfull giant his only weakness so happens to be Siegwards unique Broken blade sword, "The storm ruler"
When you enter the boss room an cutscene will immediatly trigger demonstrating you Siegward (Not as a phatom but as himself, so death is permanent) facing yhorm.
At the left of Yhorms throne you will see an item which is the STORM RULLER (remember do not rush to pick this item up) which will have a speacial property in this fight-


If you wish to sit down and take a drink with Siegward again do not, DO NOT rush towards this item.
As Yhorm will immediatly stop paying attention too you and head towards siegward.

Our Onion friend may take a few hits but will die very easly if you let Yhorm have his Window of Attack

3- Distract Yhorm
You may not be able to do any damage towards Yhorm but Siegward will take care of the boss fight for you, as he will be constantly charging his speacial attack that takes huge chunks of Yhorms HP.

So just Attack and dodge his attacks, Do not stay at medium range, but instead right below his feet, HIS ATTACKS ARE MUCH EASIER TO DODGE THE CLOSER YOU ARE, each time you hear a big wind sound it means Siegward has let out another Wind attack which will stun Yhorm

4- TRUST ONION BUDDY

This entire boss fight will be just you dodgin yhorm while letting Siegward do the damage for you, Trust him fully and he will not fail you,
He will always stay at medium range from the Giant meaning he wont get hit as long you are determined to be the Center of the Attention.

5- Drink and Collect your rewards.

As the giant falls and turns into ashes you sit down with your onion friend and take a final drink with him, You can head towards the bonfire to respec the area to find at the start of the boss room, You will find the "Catarina set" along with the Storm ruller and Sieg´s Shield.