Stemming trims inflectional endings so that related words meet at a common root. A full stemmer like Porter’s has dozens of rules; yours needs only a few to show the idea. Apply them longest-suffix first: strip -ing, then -ed, then a trailing plural -s.

The interesting cases are the guards. Only strip a suffix if enough word remains (so is does not become the empty string), and never strip the -s from a word ending in -ss like class, or you would mangle it. This crude stemmer will sometimes over-trim - that is a known trade-off, and consistency matters more than linguistic perfection, because the same rule runs on queries too.