I had to mess with the neck angle on my ESB-4 for a non standard bridge. Its not what I'd call super difficult, just need to be slow and painstaking, and I suggest not use sandpaper. I used new Stanley knife blades as scrapers, and a caliper and a little baby steel set square such as are sold for modellers.
Scraping the surface with the blade at 90 degrees to the wood gives you much more control and makes it way easier to keep the surfaces flat. With sandpaper, even with a block, I find it awfully easy to end up with a curved face. Perhaps the trickiest part is to establish whch are the correct surfaces to use as a datum. This involves much measuring with the calipher and much testing with the square. Make sure you check all surfaces are flat too.
With the scraping I usually scribble soft pencil where i want to scrape and check again when removed.
Long job, do it in a comfy chair with dust sheet, but I find the scraping much more relaxing and satisfying than sanding.
Caveat. I have no training, there may be better ways to work, but it seems much safer to me because its slow and controlled, and because he frequent checking required to renew the scrape here scribble makes it hard to go too far as is easy to do with plane or power dander







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