Summoners War: Chronicles

I seem not to understand the process of the guides available..

I been searching for a bit more specific guide on how to upgrade my equipments substats without using refining catalyst..

do I need to upgrade them to +12 before I use succession?

using +12 both equip do not match so meaning 1 should be +13?

and the formula is:

D1+D1=C1? or D2?

D1+C1=B1? or D2?

do the same process with equipment substats upgrade also applies on runes?