The most common mistake is to shoot too low down, a neck shot should be in the upper 1/3 and a fairly frangible bullet is an advantage.The effective area is actually quite large, nothing like that tiny little line on the illustrations you’ve seen, its about the size and shape of a pint glass on a sika or roe buck. Anywhere in the centre of that area will do and you’ve a lot more leeway than is generally acknowledged, you dont have to hit the spine to break it. A lot of neck shot animals will be unconscious but still alive, it can take a while for blood pressure to drop and shut the brain down, I use the knife to bleed them out but make sure its not able to thrash about before you get in close, any doubt ( or if the head is up) shoot again.
If you do neck a deer, dont hang about, get up there ASAP, if you didn‘t break the spine it may well recover enough to get up and depart.
Last tip would be to carefully mark the deers position before you fire, they often drop so fast you dont see it happen.
Neck shots are the marmite of stalking, love it or hate it with very little middle ground, personally I love it, its not perfect but then again no single shot site is.
Best of luck.