You would write it in the casual form. You wouldn't write it informal because it would sound too unprofessional. And making it formal, well, some people wouldn't understand if you were to write in the formal way. They wouldn't understand the words. So just the casual way would do.
<span>either
C.Before
beginning your project, make sure you purchase: all the items you will
need colored pencils, drawing paper, and a ruler.
or
</span>
<span>A.Before
beginning your project, make sure you purchase all the items you will
need: colored pencils, drawing paper, and a ruler.</span>
This sentence does have an error--a grammatical one.
<span>When the museum authorities investigated they found several mysterious crates in the basement.
read the sentence out loud. doesn't it feel like a run on? there isn't any pause within it, but it has a sequence of events: the authorities investigated, the authorities found several mysterious crates. you'll want to insert a comma between "investigated" and "they"
</span>
When the museum authorities investigated, they found several mysterious crates in the basement.
read it out loud again. you have a natural pause that helps you better understand the events and you have a comma separating the two ideas.