I am not claiming this is the best failstacking method but it is my own.

Table of Contents:

A. What are failstacks and how are they helpful?

B. How to get failstacks?

C. Things to prepare?

D. Failstacks I use for each enchancements

E. The Method

F. Item Popped to +15?

DISCLAIMER: This failstacking method is only up to 30 FS which is already a TRI stack in my opinion. Getting a TET stack is too RNG.

A. What are failstacks? And how are they helpful?

Failstacks give an increased percentage of your chance to succeed for enchancing. Failing to enchance an item after PRI will cause the item to degrade and lose durability, however if you are enchancing accessories it will break. The only way to get around this is using cron stones. Cron stones will equal to the value of your current item's max marketplace value so I do not advise it unless you're softcap and have 20b in silver extra. You should use failstacks to lessen the amount of silver you spend in repairing your items or buying new accessories. 

B. How to get the failstacks?

Failstacks are gained by obviously failing an enchancement.
+5-+15/+7-+15 = 1 fs added

+15-Pri = 2 fs added

Pri-Duo = 3 fs added

Duo-Tri = 4 fs added

Tri-Tet = 5 fs added

Tet-Pen = 6fs added

Accessories = always 1 fs no matter the enchancement level.

C. What items to prepare?

NOTE: Reblath can be bought from the Velia blacksmith for 12k-15k silver
Cheap Route (More RNG based because reblath sometimes pop +15 at 0 fs)

Bring: 1 or more +14 Reblath armor/helmet/shoes/gloves (Make it yourself just smash it to +14)
All your blackstone armors.

Relatively Cheap Route (Mixing green gear will allow an easier way to get the failstacks but is more expensive)

Bring: 1 +14 Reblath

1 +14 Cheap and Abundant Green Armor

1 +14 Cheap and Abundant Green Weapon/Offhand

All your blackstone armors 

Decent amount of blackstone weapons.

D. Failstacks I use for each enchancement

Now personally this is the failstacks I use.
Green Gear:

0-15 = Smash
Pri = 10-20 FS
Duo = 15-30 FS

Tri = 27-40 FS
Tet = 40-70 FS
Pen = 70+

Boss Gear:

0-15 = Current enchance level + 3 (+10 = 13 FS)

Pri = 15 FS

Duo = 20 FS

Tri = 30 FS

Tet = 45FS

Pen = 100+
 

E. The Method

Cheap Route

Smash your +14 reblath from 0 to 30 FS or how however much FS you need.

Max is 30 FS

Relatively Cheap Route

Use your Reblath up to 10 FS

Use your green armor up to 20 FS(USE DURABILITY MODE NOT ENCHANCE)
Use your green weapon up to 30 FS(USE DURABILITY MODE NOT ENCHANCE)

TET/Pen Stack? You'll get there eventually failing your tri and tets.

F. Item Popped to +15?

Now there are 2 ways to downgrade your failstacking item.
1. Oldschool Negative Karma - 

You have to make a Negative Karma character. Have it equip your +15 failstacking item and die from the town guards repeatedly until it downgrades.
Pros - Free, Get back 1 blackstone when it downgrades

Cons - It takes a little preparation and based on RNG can take you a long time

2. Degrading NPC -

I know only of the Velia and Heidel NPC but they are at the church. Talk to them and pay for your item to be instantly degraded to +14.

Pros - Fast/Instant

Cons - Cost 100k silver and does not give you blackstone back.

Me personally if I have alot of blackstones I'd rather go to the degrade NPC but once i am at 20 blackstones left I'll do neg karma method.