I don't understand this about affixing

[Kole]

Well, I understand that you can upgrade the tiers with the same one, and need that soul item thing for that, but do the items have to be the same rarity? or just same slots?

[Qualia]

Nope, just same slots or higher.

[oratank]

you can use same unit to make affix this will rise success rate about 15% of base success rate

[Buckets]

http://srpgp.supersanctuary.net/dudu/

[TaigaUC]

As in, if it's the same name of weapon or unit, you get that bonus.

For example:
If you want to affix Chain Sword, and you are using Chain Sword as fodder, you will get a bonus to success rate.

[GHNeko]

http://srpgp.supersanctuary.net/dudu/

That doesnt teach you how to affix tho lol; which is what OP is trying to learn; how to affix.