How to keep sideburns

It may be a stupid but crucial question.

Every now and then I manage to ruin my a sideburn of mine (make it shorter by accident), then, I have to make the other one shorter too.

How can I avoid ruining my sideburn from the beginning? And when it grows again, I see it has stubble and hair from the head, so can I still shave the stubble there and the hair head will keep growing to the sideburn? Otherwise it will look ridiculous = a sideburn made from stubble.