Here's what I think I know:
C'est is used at the beginning of a sentence for what comes after is a noun, modified noun, proper noun, or stress pronoun. So basically I usually look for an article like un, une, le, la, and so on.
For il/elle est at the beginning of a sentence, you use it if what comes after it is an adjective.
So when I'm trying to describe an object, like a book. Would this description be correct?
C'est un livre. Il est orange, rouge, et bleu. C'est en papier. Il est assez grand. Il est rectangulaire. For this one I'm not sure whether it's “il est” or “c'est” because I think I heard someone say “c'est” before but rectangulaire is an adjective.
So my question is when describing a shape of an object, do you use “c'est” or “il/elle est”?