With the help of his horn a new-born changeling tears open his cocoon and emerges into the world, fully grown.
Though changelings only need to feed on love to survive, they eat other things too in order to produce body liquids which they excrete through the holes in their legs. Those are used to fortify the hive, form cocoons for the eggs, and various other things, such as decorations and traps. Additionally they allow the changelings to walk on almost any surface. The Miasma hive prefers rotten fruit and the occasional carcass, which is what gives it its particular smell.
The most common trap for gathering love is to lay out some sticky liquid in a remote area and wait for a creature to walk into it, then send in a disguised changeling to rescue the creature. Thus gaining its trust, the changeling will begin to deepen the relationship and feed on its love. Usually multiple changelings take turns playing the role and feeding, thus speeding up the process of transforming the creature into a husk.
Most changelings are unable to transform into something beneath their own body mass. The only exception to this (beside queens) are changeling commanders, who thanks to higher willpower even manage to transform themselves into little fillies. They are usually the first ones to infiltrate a town when the changelings plan to “stock up” on love, because no-one suspects children when it comes to reconnaissance.
(“Stocking up on love” would be something like what happened during the season finale)