Before I start, I would like to make a disclaimer that building your own failstacks can be extremely expensive to start and it does require a lot of Blackstones (both normal Blackstones and Concentrated Blackstones). Building failstacks is also super RNG and your sanity will most likely decrease after this process. With that out of the way, let's begin.
This is what I do personally, I have about seven +14 Reblath Shoes and about fifteen PRI - TRI Grunil Helmets in my inventory. All of these are used for making failstacks. I recommend building failstacks in Velia as the blacksmith sells Reblath and the priest is also located nearby. Refer to screenshots below for locations. Also, refer to Q&A section below for further clarification.
Basically, the process goes as such; at 0 failstacks tap +14 Reblath Shoes until you reach 21 failstacks. If at any one point does your +14 Reblath becomes a +15, you can downgrade it at the Priest in Velia for 100k silver. Once you have successfully reached 21 failstacks, start tapping PRI Grunil Helmets. Each fail at PRI will grant you +3 failstacks. You will want to attempt to fail these PRIs until you reach 30 failstacks. At this point, you will now attempt to tap DUO gear. Each fail at DUO will grant +4 failstacks. You should attempt to fail these DUO gears until you reach +42 failstacks. After you reach +42 failstacks, this is where you attempt to tap your TRI gear. Each failed attempt from TRI to TET will grant +5 failstacks. Attempt to fail your TRI gear unitl you reach +87 failstacks.
If you run out of durability, use the base version of the gear to repair. DO NOT USE MEMORY FRAGMENTS. Memory Fragments are strictly for Boss Gear.
Keep in mind that usually people attempt TET at +60 failstacks. So you have a choice if you want to continue building failstacks past +60 with green gear or boss gear. It is probably wiser to use TRI boss gear at +60 failstacks but this is completely up to you.
0 - 21 FS: Reblath shoes
21 - 30 FS: PRI Green Gear
30 - 42 FS: DUO Green Gear
42 - 87 FS: TRI Green Gear OR TRI Boss Gear at +60 onwards
87++ : Just start attempting TET Boss Gear
Location of Church and Velia priest:
Q: Why use +14 Reblaths?
A: Because Reblaths are dirt cheap and are easily repairable at the blacksmith. 1 Reblath = 10 durability and it is definitely more economical than using Memory Fragments.
Q: Why use Grunils instead of any other Green Gear?
A: Grunils are reletively cheap and most importantly, RELEVANT. What do I mean by that? While building FS, you are most likely going to pass a TET Grunil in the process. By being relevant, you can sell the TET Grunil on the CM for a hefty profit. Another reason I choose Grunil over others is that the base +0 Grunils are more widely in stock as compared to- let's say Strength Helmet of Heve.
Q: Do you regret investing about 700m+ into building failstacks?
A: No, I don't. I feel that this investment is definitely worthwhile as this method can produce +40 and even +50 failstacks with reletive ease. It puts me at ease knowing that I can simply generate failstacks should I ever need them.
Q: The enhancements always pass before I reach my desired failstack! This is rigged!
A: Yes. End of the day, this method does require RNG. Keep going and don't give up. Good luck.
If you have any other questions, you can whisper my family name ingame: Megumin. I should be on most of the time. Best of luck to you.