The max you can fort up to is +18. You can use various items such as Sage Stones Master's Fortified Inscription and Lucky Fortification Clovers. Depending on what you're forting, depends on the scrolls. Defense scrolls are required for armor and Attack scrolls are required for weapons. Grade S Attack/Defence are require for items +16-18, though that's ireleveant. at this point. As I dont fort with SS's or LFC's, I'll give you the MFI's on which you can fort up to
MFI1 to +7
MFI2 to +10
MFI3 to +13
MFI4 to +15
MFI6 to +17
MFI7 to +18
Once you've obtained the require materials. You go to a fortification Officer in either Ilya Jale or Kaslow. They usually have a hammer as their symbol on the map. You place the armor/weapon in the top right hand slot. The scrolls in the top left, and the MFI/SS/LFC in the bottom left.
I have found with my experiences it's easy to fort up to +5-9 without any assistance of MFI's/SS's/LFC.
There is also a guide here
for some more information. Its a little out dated but relevant nevertheless.
Hope this helps~