Son de

Question

Would the literal translation be: “The dolls are of plastic.”?If so, how would I know when to use a word for “made”?________ muñecas son de plástico. The dolls are made of plastic.

Accepted Answer

Hola Melissa 
We often use the preposition "de" for description, so, if in English you say "a plastic doll", in Spanish we say "una muñeca de plástico". Here's a lesson on this use of "de".
You can also make it more literal and say "made of plastic" by using "estar hecho / hecha de + [material]:
Esta una muñeca está hecha de plástico. 
This doll is made of plastic. 
Here's another lesson on this other specific topic. 
I hope they help.
Saludos cordiales
Inma