How to get more Slots?

I'm trying to figure out exactly how to add more slots onto a weapon so I can add more augments to it. I'm not too sure and tried searching but it seems nobody gives a concise and through description and hoping someone would help.

You do it through what is commonly referred to as "up-slotting". When you go to affix, there is an option for an "Extra Slot" and using the Extra Slot will increase the number of Slots, but it also decreases the chance of success (the higher the number of slots, the more the chance of success drops when up-slotting).

@AndrlCh thank you. Is there best spots to farm souls and whatnot?