Amateurishly is the adverb: it tells you how something was done or made, and the answer is “not with skill.” A film may be amateurishly shot; a speech may be amateurishly put together. Like amateurish, it judges the work, not the person’s pay. It is a little heavier than badly or clumsily, and it often carries a picture of someone trying, but still looking untrained. You will hear it more in reviews and careful talk than in the playground. You attach it to a verb, not to a person as if it were a feeling.