For determination of the certification stall speed it's normal to trim as required by the regs, power to flight idle, re-trim to that speed at idle power, then decelerate on the primary pitch control at 1kn/s. In virtually any aeroplane (maybe not the odd high performance glider or motorglider) you'll be descending for most of this.
There are of-course many other stall tests you need for handling / cert.compliance purposes, but that's the one for determining Vs (an in particular whether Vso is low enough for compliance with the design code stall speed minimum.
G