I've built one of those LaBelle kits myself.
Along with MWB's post above, you can also order a roof rounding kit from LaBelle, which Makes rounding the end much easier. They also have a great tutorial on their website. Use power tools, they're your friend on an o-scale kit. I use a DA sander myself. Lastly this thread may help:https://ogrforum.ogaugerr.com/topic/la-belle-woodworking-o-scale-passenger-kit-a-true-craftsman-kit?