Elden Ring - How to summon other players to help fight bosses?

In the last Elden Ring video, we discovered that it's almost impossible to beat the Castle Morne boss on your own. Therefore now we need to go back to the start of the castle and travel another route to find the "Furlcalling Finger Remedy" item so we can summon another player.

As we proceed this way, you will find groups of enemies (Misbegottens and Godrick soliders) fighting each other. With the Lordsworn's Greatsword I have equipped, I found that you can easily hit multiple enemies at once. This is very important, because if you only focus on one of them, the others will take you down, so it's best to just defeat all of them with a single attack.

Items picked up in the video:

03:39 Fire Grease
04:00 Claymore
06:32 Iron Cleaver
07:11 Lordsworn's Bolt
07:27 Steel-wire torch
08:06 Furlcalling Finger Remedy
11:15 Grafted Blade Greatsword

After finally finding the finger (I was about to give up when I couldn't find it, then I found it), I fast travelled back to the site of grace just in front of the boss to start the boss fight again.

As you use the item Furlcalling Finger Remedy in front of the final boss portal (before you go in), you see a gold summon sign in front of the portal on the floor. Touch the sign and you can see that you can summon Castellan Edgar. Initially I thought this was another player but it's actually just an NPC. However, it should be the exact same way to summon another player.

With Castellan Edgar's help, it was almost too easy to defeat the boss. Now you have to repeat the first part of the dungeon to find Edgar again to let him know you defeated the boss.