으면 넣다

if X, put it in

A conditional "-(으)면" plus 넣다, "put in, insert". Most lines are cooking or packing instructions: add whatever else you want, if you've only got the peels put them in and boil, I'd have put in a meaningful number if there'd been one, I'll put in anything you need. It also covers the negative rule — you mustn't put it back in once saliva has touched it. 넣다 is putting something inside a container or a mixture, not placing it on a surface.