The above image is where I start. I use a nail file, sapphire. As it cuts fast and leaves a nice finish.
I first just file so that all layer lines are gone. I do the same to the underside of the tongue. You may have to go back and take a bit more after all other steps listed.
Next step and the one you need to take care in doing. You need to blend the transition between the taper and the flat section. See where my knife is pointing. I use a small pen knife blade. Anything will work. Make sure you get underneath the flange areas. What this does is creates a better taper for the blade to follow when putting the cap on.
I then move my attention to the base. As you see I have the handle mounted. Speaking of mounting the handle. Most of you in the U.S. have come to find out that sourcing the hardware is next to impossible. I ended up having to buy 50 of each the metric flat head and jam nut from McMaster Carr. I have plenty now if you live in the US PM me we can work out shipping you some.
Back to the filing. Do the same thing as you did to the cap. File the layer lines and bumps down only to be smooth. Now is the time for a dry fit. You may have tinker a bit. You should be really close.
All the above takes maybe 5 minutes. Once you have the cap fitting. This is important. It should not be a loose fit. Snug is much preferred. As the blade is held by the tight fit of the cap to the base.
Now is the time to try it with a blade. Important that you use an old blade for this. When you slide the cap over the blade for the first time. You will shave a bit of material from the cap. This is normal from my experience.
Open cap install a new blade and enjoy your shave.
I hope this clears up some things for you all.
Tom