Can’t make dwarwes to dig stairs in DF

This depends on how you built the stairs.

If you designated them and a miner constructed them, you can designate the stairs with "Remove Up Stairs/Ramps" d-z

If you constructed them in the build menu, you want to designate the stairs with "Remove Construction" d-n

In my experience, the safest way of doing it is to designate one level for channeling at a time, waiting for the previous one to be completed before you continue.

It requires a bit of micromanagement, but it generally works pretty well, and as long as you have a maintenance chamber at the bottom, you generally don't need the up/down staircase.

Of course, Dwarf Fortress isn't exactly a bastion of OSHA compliance, and mining is a cheap skill to train, relatively speaking, so safety can in many cases be favorably sacrificed for speed.

That said, the absolute fastest way, so long as you have access shafts on each Z-level, is to dig each level, then build a constructed floor above in the shape of the desired pit that is subsequently collapsed using a support.

Just make sure that the pit has solid ground underneath…