Using "shì" for noun + adjective + noun
Seems the answer did not help much, here is mine - it is about predicative and attributive.

My phone is new. The adjective describes the subject, this is the predicative.
My new phone. The adjective is attributive, it is linked to the noun.

If it is predicative, do not use "to be" (shì).

SIDE NOTE: I am a beginner myself and I often see the example "Sky is blue" translated as "Tiānkōng shì lánsè de (天空是蓝色的)". I have no idea why it can be like this as it clearly is predicative.