You can harden clay with heat.
Viet takes an hour to make his pompadour hard every morning.
Koichi decided to fortify his position and send his troops to Japan.
This word consists of kanji with hiragana attached. Because the hiragana ends with an う sound, you know this word is a verb. The kanji itself means hard so the verb vocab version? To make hard. You need to differentiate this from 固まる, though, which means "to harden" (the hardening happens on its own). This word means "to make hard," because you're doing the hardening. You can remember this because you want to make something hard. And to do that, you send special "mail" (める) to this thing you want to make hard, and this mail does that for you.
Since the reading for 固 (かた) is different than the one you learned with the kanji, here's a mnemonic to help you out. A caterpillar (かた) forms a cocoon. The caterpillar's cocoon is hard and the caterpillar made it that way. Keep this in mind and you too will blossom into a beautiful butterfly.