With mag out and chamber empty, get the slide and barrel lined up properly on frame to put the gun in battery as if the recoil spring were still in it. Holding the gun upside down makes this easier.
Pull the slide back to reset the trigger.
Grip the gun almost as if you were going to fire it, except push forward against the slide with your thumb to simulate the missing spring.
Decock as normal. Don't let the slide come back at all.
With your other hand, start to pull down the slide latch.
With your "firing" hand, grip the slide with fingers and frame w/thumb under tang.
Pull the slide back just far enough for the slide latch to disengage: about 1/8" to 1/4".
Push off the slide and barrel.
It may take a few tries.