Here is a picture of my 2 tools I use to make sure decals go on straight. Just to be clear I usually use them to lay masking tape that will be the guide where to place the decals. I'll often hold the tools in place with masking tape as well so I have 2 hands free to place the masking tape. In addition it can be helpful to mark on my placed masking tape where the center of the tube will be so the decals don't end up off center.
