A properly balanced bin doesn’t smell, but getting there took me two failed attempts. The first failure was too much wet kitchen scrap and not enough dry material, it turned into a sour, swampy smell within a week. Adding shredded cardboard and dried leaves in roughly equal volume to the scraps fixed it within days.
The second failure was forgetting to turn it; anaerobic pockets form fast in a still bin and those are what actually smell. A weekly turn with a hand trowel keeps oxygen moving through and the smell stays essentially neutral, like damp soil.
Citrus peels and onion skins are the only things I still leave out entirely, they break down too slowly for a small apartment-sized bin.