If you already have the pieces and maiko's comissioned map you can skip to the next section

You will first need to get four pieces of the map, plus Maiko's map. I wont give much detail here because there are lots of guides online and on the uesp sites and videos. I will focus on finding the flask after you got all the maps you need, because I havent seen no one talking abt this besides the redguard companion, page 164 (you can find it for free on the internet archive).

Talk with people (some people are not as easy to talk, like the snake charmer will only talk if you tip him with gold), eventually you will hear abt the flask of lillandril. I recommend talking with everyone you can and exhausting all dialogue options. Eventually you will find out from the argonian barkeep that Crendall, the drunk breton that wanders the town square has a map piece. He stole it from Trithik. Talk with thrikit and with crendall, you will then convince crendall that his map was stolen, he will call you a liar and tell you to go away, but he will check his hiding spot after you get some distance. Follow then while keeping a distance and you will see that he buried it below a tree and get it with a shovel that you must buy from gerrick's goods.

Also now is a good time to go to j'ferr books and buy his book abt the flask of lillandril. Then go to maiko's map and comission him a map of the flask of lillandril, in exchange he will ask you the book you bought from j'ferr and some time to make it.

The second piece is inscribed on the back of the league insignia that you recieve from the smugglers den in the contacting the league quest.

The third piece you can find after learning from falicia in the mages guild that one of the pieces was inscribed in a silver boat. So you will logically head to the silver smith hoping to know more about it, she will tell she never heard of it but inherited a lot of junk from the old silversmith, so you will try to enter her house next to try to find the boat and find two agressive imperial guards ransacking the place (the empire wants the flask to invade summerset isle, it will help dispell the high-elves magic and make the invasion easier. Fight the guards and report back to the silversmith, she will give you a key and tell you to search her storeroom, next to her house. Go there (the door is on the opposite face from where the door to her house is but its still the same building) and check the two chests inside. The one farther from the door will have the silver boat.

the fourth piece you can only get after getting back from the league hideout. Talk to falicia in the mages guild, she will tell you abt the soulgem and izara probably as a prisioner of the necromancer, since she foolishly resorted to his help without the flask to protect herself. Then she will tell joto knows more, but he is in prision. At this point the archmage will turn you into some nasty little gremlin critter. Go to the prision next to the palace and through the open window at the right of the main door. Find and talk to joto, the guards wont mind you. You will need to complete a word combination puzzle and spell cha-eng-mee-bak (change me back) and joto will make you human again. He will give you then the last map piece.

With all pieces go back to falicia and she will give you a health potion.

Now go back to Maiko's and get your map you comissioned. You are ready to find the flask

Now this is what this guide is all about. Piece the four pieces together. There is no way to do this in game, but you can trace the complete map in your head or note it down in a paper. You should be left with something like this:



After exploring saintsport (which has its name after the saint it homages in the shrine at the center of it, leki) you will find the shrine of leki in the middle of it (you know its leki because there is an image at the ceiling of the shrine that is the same found in the redguard history book you buy from j'ferr, which says its leki. Now the map you got from maiko says that leki is the start, so you must place your character at the center of the shrine.



Now here is the catch, in maiko's map, that has a wind rose telling where is north, the long tree (that is also long gone by now) is to the left of the shrine, but in the map it is at the north of it, so the only logical thing to deduce is that the flask map is with a wrong orientation, pointing west instead of north:




the flask map with the correct orientation, now pointing north at the top

The number in each line represents the number of steps you must take in each direction. Activate your compass and press tab to deactivate autorunning, as the steps must be done walking.

-Take 12 steps west
-Take 25 steps south
-Take 31 steps west
-Take 5 steps north
Take 20 steps west
-Take 4 steps north

At this point you will have reached the location of the flask, take your shovel and dig.



Open the chest and get the flask. Now you have the mighty Flask of Lillandril and can be protected from N'Gasta while trying to rescue your sister. Next step is to go back to the isle of N'Gasta and confront him, with the help of the flask as protection.

I created this guide since I havent found anywhere the explanation on how to follow the torn map pieces to get the flask. Online I only found the exact location of the flask, but not a guide on how the maps work.

What explained everything to me was the redguard companion, so credits to it for the solution

The screenshots I took from there, from my own game and from the internet (google images) and screenshots from youtube, without any permission from anyone but I dont take credit for them, so all is well.

The only credit I take is writing this guide on steam and popularizing the solution I found on the companion book.