I'll try to use an example to explain this. Imagine a person that has not slept in two days. This person has been working hard for hours and hours. This person has been wearing the same clothes for a week. This person has messy hair, red eyes, and dirty clothes. When you see this person, you would say "you look a sight." OR This person might say "I must look a sight." This is kind of an old saying and is uncommon to hear. You can also say "what a sight!" when looking at something beautiful. Imagine looking down from the top of a mountain onto a forest with a river running through it. You would say, "this is quite a sight." Finally, there's also the saying "you are a sight for sore eyes" which means "I am happy to see you." The saying means that your eyes are sore because you are tired, or you have had a bad day. But then you see someone who makes you happy, so you tell that person "you are a sight for sore eyes." I hope this helps.

You would not use them together, only separately. We would use it if someone looks bad or unusual For example, your hair is a mess and your clothes are all crumpled, someone might say Wow, don't you look a sight.