Forging is done in 2 ways with 2 different types of items.

Regular equips (equips you buy in the game that are produced from compounding, quests, and shops in game)
When you go to the IM forging room you select the equip you want to forge and without any forging it will say +0 for the equip so you click confirm. If the forging is successful your equips will say +1. Forging from a +0 to a +1 have a very high success rate with little to no chance of failing. You can continue to forge your equips in the forging room but every time you succesfully forge and item your chances of having a successful forge will decrease. It costs you 3 points per forging attempt even if you fail so forging can become very expensive very quickly. Someone I know spent 100 points and forged a pair of boots to +6 while another friend spent 100 points and ended with a +4 forged item so depending on how lucky or unlucky you are forging can get pretty expensive.

Item Mall equips(IM equips - equips you purchase with points from the IM [archangel wings, lord sword, energy halo, etc.])
IM equips can only be forged using Strong Scrolls. With the IM equip in your inventory go to the forging room in the IM (like you do with regular equips) only this time when you place the equip in the IM it will say +0 and when you click confirm it will ask for 1 strong scroll to enhance the weapon (not 3 points). Enhancing IM equips with strong scrolls is always successful so you don't have to worry about failing. Every time you increase your IM equips it will require more Strong Scrolls though. So to get an IM equip from +0 to +1 it will cost you 1 strong scroll. From +1 to +2 will cost you 2 strong scrools. From +2 to +3 is 3 strong scrolls and so on. Currently IM items can only be enhanced up to +8 but even then you would still need 36 strong scrolls and strong scrolls can only be found in the IM or obtained as a reward from the Cursed Palace event.

So as a recap:

You can not use strong scrolls to forge regular equips. Strong Scrolls can only be used to enhance equips from the IM.

forging regular equips from the game requires 3 points per forge even if the forge fails

with each successful forge your chances of having another successful forge are decreased

IM equips can only be enhances with strong scrolls (with the current update they can only be forged to +8)

IM equips do not have a success or fail rate. Forging IM equips will always be successful

to forge IM equips:
from +0 - +1 (1 strong scroll needed)
from +1 - +2 (2 strong scrolls needed)
from +2 - +3 (3 strong scrolls needed)
from +3 - +4 (4 strong scrolls needed)
etc.