"so shall you observe to do": You may not do so (i.e., you may not remove a plague-spot deliberately), but you may tie the (shoe) thong around your foot, and you may place the (carrying) pole on your back, and if it is thereby removed, it is removed (and there is no liability).
(Devarim 24:9) "Remember what the L-rd your G-d did to Miriam": What does this have to do with what precedes (i.e., plague-spots)? To teach that plague-spots come only because of slander, (Miriam having slandered Moses regarding the "Kushite woman" that he had taken). Now does this not follow a fortiori, viz.: If Miriam, who had not spoken in public of Moses, and only for his benefit, and in praise of the L-rd, and to the end of the propagation of the world, was thus punished (with leprosy), then all who speak in denigration of their neighbor in public, how much more so!