what should i do to +12 weapon should i fortify everyday, or should i stack stones?
There is some luck involved but usually stacking gives the best result simply because it gives you better odds.
Obviously using 3 ES5 + 25% ammy (+Divine Ammy) = best percentage.
But some players also like to split it up. Such as if they had 6 ES5, they'd rather try it 6 times with 1 ES 5 each time instead of 2 times of 3 ES5. This technique does give more chances but also reduces percentage of success by a lot.
Personally, I like higher percentage.
I've gotten both my +12 so far using 3 ES5 and 25% ammy. I normally stack a few before I try though so I can have multiple tries (like get 15 es5 so I can have 5 tries).
Use the guild armory to your advantage! Make sure your contribution is high enough to get guild buff. VIP does give some increased % as well.