Beware! many of the ready made steels I have seen being sold for this purpose do not work well, if at all. Do not buy a steel without trying it first, if the vendor will not let you try, treat it as suspect.
The flint is important too. I have found that the darker black it is the better it strikes. There
is certainly evidence that this dark flint turns up as archaeological finds in many areas where it is not native long past the stone age.
A sharp edge works best and depending upon your preferred method this can be acute or almost square.