There's no official guide yet, but feel free to let me know which ending you're missing (by its number or picture), and I'll give you a hint!
Viewing post in Bloody Flower of Teyvat [Genshin Impact Fangame] comments
These endings are actually secret and the hardest ones to get! :)
Here are the hints:
For Ending 1: You need to unlock all endings for any other character (Wanderer, Xiao, or Lumine, since the other routes aren't fully complete yet).
For Ending 2: You need to agree to bind your fates with him. To do this, clear all of Kazuha's April 1st endings, and when you end up in that unusual space, choose the option to "Bind fates".
For Ending 5: You have to go on a date with him and choose the right outfit and food to keep Kazuha satisfied.
For Ending 10: You'll have to travel around the game a bit for this one!
First, you must unlock all 4 endings where you get killed:
Wanderer's New Year ending ("STFU tactical nuke incoming")
Wanderer's main route ("Frozen Love")
Xiao's Feb 14th ending ("Time Didn't Heal This One")
Xiao's main route ("Wasted...")
Once you have all four of those, go to the party again and drink the strange liquid (just like you do for Ending 9).