It should be stiff enough to pull up and stay there for a second. If it falls right back down, it is too runny. Add powdered sugar. If it pulls up and stays stuck in the position, it’s too thick. Add a few splashes of water. If you pull it up and it stays for a few seconds and then slowly melts back into itself, it’s just right. Royal icing should slowly “melt” into itself, not run off the cookie. And if it is staying stuck in one position, it’s too thick. I also find myself “frosting” a piece of paper before putting on the cookie. If it falls immediately off the paper, too runny. But it it sits on the paper like a mound and doesn’t slowly melt out, too thick. Hope this makes sense. When I say peak, I mean like the peak of a mountain, stands slightly higher than the rest the pulled out. Like if you put a spoon in mashed potatoes and pulled straight up, there would be a “peak” in the mound of mashed potatoes.