- This is a frozen pack. To cook, bake for 20 minutes at 200 degrees celsius.
- This is a frozen pack. To cook: bake for 20 minutes at 200 degrees celsius.
Is colon correctly used here?
If you are writing ordinary sentences, the colon is incorrect. The rules require the text to the left of the colon to be able to stand on its own as a grammatically complete sentence. #1 is the correct sentence.
However, if you are writing instructions (something you might find on a package, for instance) then you may adapt the rules to suit your needs. It is quite common for instructions to use sentences like #2. The correctness would be determined by the manufacturer, perhaps with reference to an in-house style manual.