Questions on drop sets

[Bjwr]

Hey just trying to get my head around the concept of drop sets, can anyone help me out,
Say I'm training chest
Decline BB press 4 sets
Incline DB press 4 sets
Flat flyes 3 sets
Incline flye 3 sets

Now is this right or wrong say. I'm gonna do a drop set on decline would it be
4sets, drop weight, another set, drop weight, another set.
So in total that actually 6 sets
Or do I do my first decline set, drop set he last 3 to total 4sets?

Hopefully it makes sets!
It's just an example I would mostly only drop once.