I hope this is not only me, but I used to struggle a lot with placing small decals using my big fingers. The fingers tend to get in the way, and cause the decal to end up in the wrong position.
So here is how I now do it:
This might not be the smallest or most difficult decal to position, but it's a nice demonstration.
Peel off the backing and position the decal on the tip of your modelling knife.
After some practice you will find how to position it best with respect to rotation and how much it will stick to the knife.
Here we go, no fingers to keep away, only the very thin blade of the knife, making it easy to position the decal.
Position the decal, and press down with a finger.
And there it is, spot on. (pun intended)
Written by Larbut