A complete sentence does not have to precede a colon.
1 For example:
2 John: he was a real good friend until he stole my girlfriend.
3 Texas: It’s Like a Whole N'other Country.
3 Butter, sugar and flour: this is what I need to buy at the store.
4 Butter, sugar and flour: these are what I need to buy at the store.
5 Instructions: Assemble each light, hang the lights on the tree, plug in the power cord, turn the power switch on.
However, in your example:
I want the following: butter, sugar, and flour.
I want the following is a complete sentence. The subject is I and the verb is want. The direct object is the following.
Note: grammatically it is a complete sentence. Semantically, it may or may not suffice. For example, unless the writer or speaker says what he wants, it comes close to being an incomplete thought, which might wreck the idea that a sentence expresses a complete thought.
But, it could be said in real life, as in
I want the following and the person is going to tell you what he wants, but then the phone rings and he doesn't get a chance.