I would use commas, as there are really two clauses there. The second one is incorrect. - Be careful, do not drop it - Be careful, do not let it fall - Be careful, do not knock it off

Hi I would subsitute the words "dont" and "doesnt" for "do not". In spoken english the shortened version has more emphatic qualities. For instance "Be careful, dont drop it!" "Careful it doesnt fall!" "Careful you dont knock it over!" Remember to use exclamation marks in written english to emphasise the urgency of the statement too