A predicate consists of a verb and maybe objects (in traditional grammar). A simple sentence follows Subject-Verb-Object order. Your examples are simply missing a verb. The gerund is a noun form of a verb, so it cannot act as the verb in the sentence. The whole phrase (Gerund + Object) is a noun phrase.
Because you seem to have a list of instructions, those can be formulated using the imperative: "Prepare the raw materials!". The subject is implicit in this form and this isn't a real sentence.
Alternatively you can use the passive voice to form a sentence: "The raw materials are prepared". The active voice would need an additional subject to perform the activity.
Of course, the way you have written it is common, too, but that gerund form sounds odd in the second example, because you use the gerund like verbs, as if it should be a sentence. If you have a list, "then" would usually be another entry in the list ("3. Put ...").