Help me with the next step

I am level 90. I have random legendaries as my equipment. And I have not been lucky with drops and i need help with item farming .

How do you farm an exact legendary item.

For example I want to farm inferno set. http://prntscr.com/asploj (It’s from the legendex excel file)

Where should i go? For example the amulet shows that you need to go to Fire 20. Is it every floor after 20? or every 20th floor?

farm at fire maps