Usually, you ignore it. Simple systems in home/office conditions, with proper supply bypassing, "never" have noise margin problems. That's one great thing about CMOS.
If you have a big complicated system, sitting inside an arc-welder, you do have to worry about noise margin. But noise-resistant design is probably too complicated to explain on a forum.
Find a copy of Lancaster's "CMOS Cookbook". It is really all you need to know about CMOS.