Hello Len
With difficulty it seems. I haven't lined a vechicle for many years with Fox decals.
Before my E4 (below) my last item was a maroon Clacton 2 car EMU. I had similar difficulties you have described at which point I tried to be clever and use the Microscale Microset and Microsol solutions. The decal just creased up and I had to start again this time lining in sections without the Microscale solutions.
The later Fox decals now seem to accept the Microscale solutions and I believe they have altered their backing film. (Apoligies in advance if you already know what's coming) Microset appears to be a very weak acetic acid solution and acts as a wetting and setting agent to help the decals slide over the model. Microsol is a softening solution used once the decal has been located and softens the decal to draw it over irregular surfaces such as louvres, rivets etc. It does initially crease the decal but it then dries smooth over the irregular surface.
After my early disasters I bit the bullet and used Fox decals (over a year ago) for the BRITISH RAILWAYS lettering and numbering on my O scale E4 using the Microscale solutions. The Microsol 'drew' the numbers across the bunker panel as can be seen.
I believe there are other brands of decal solutions on the market and I only use Microscale as I also model Canadian and American railroads in HO.