Write some simple sentence constructions on the board. Ask the class to point out the verb in each sentence. Ask the class to describe what a verb is, i.e. a verb is a "doing" or "being" word. Get the class to give you some more sentences that contain verbs.
Write some sentences on the board that contain adverbs. "The boy grinned nervously," for example. Or: "The girl spoke loudly." Keep the sentences simple. Get the class to point out the verb as before, then ask them what type of words "loudly" and "nervously" are.
Explain that adverbs help to describe a verb. Adverbs can tell us how something happens, when or where something happens, how often something happens or they can make the meaning of a verb stronger or weaker.
Ask the class to try and think of some sentences of their own that contain adverbs. Come up with some more as a class if they are struggling individually.
Say that adverbs can come before the verb or after the verb. Write out some simple sentences twice with the adverb in different positions and get the class to tell you which is correct.