It seems a bit of overkill, but I find that the only way to do it successfully is to use both hands: one to squeeze the power button and the other to squeeze the volume down button. If I try to do it one-handed I always fail.
I somewhat miss Android 6.x ability to take a screenshot from the Now on Tap menu.
But, wait - I just found something new!
From anything other than a home screen, open the Google Assistant (you know, long-press the circle at the bottom center of the screen), then tap the "Share screenshot" button that appears. Mail it to yourself, "Save to Drive", or do whatever else you may have planned for it.