If you're converting monsters from other games or working with a clear concept (a new kind of dragon, for example), you can just start selecting descriptors from this list, making up your own where necessary. Should you end up with more than three (which is easy to do using this method), just ask: does this describe every single creature of this type? If the answer is no, it probably shouldn't be one of the descriptors. Still, note that you considered it for later, as it may come in handy when assigning weapons, conflicts, and instincts.
You could also approach that list without a concept and put triads of descriptors together until they evoke a creature of interest. This is great for making things up on the fly or when you want to throw something new at the party. The thing to be cautious of (and this is true in general) is to avoid putting too many like descriptors together. You want a variety that suggests how the creature defends itself, travels about the world, what purpose it may serve rather than three ways it kills all humans.
Alternatively, you can generate or further define a rough concept by answering some questions. These were heavily inspired by the Nature question in the rulebook, and are by no means exhaustive. Ignore any that don't apply and make up your own!
- When humans invade your home with fire and steel, do you guard it fiercely or retreat further into the wilderness and start over?
- Do your roars of victory echo through the night, frightening the dead awake or do you brood silently, plotting your next raid in the shadows?
- Were you born in the volcanic forges of smoke-plumed mountains or hatched from the icy hearts of glaciers? Somewhere in between?
- Are you the only one of your kind, the likes of which the world has never seen or one in a rising tide set to swallow the mountains?
- Do you take comfort in your trinkets and treasure deep underground or do you stalk the world constantly searching for more to covet?
- Is your skin harder than stone or does it bruise like an apple?
- When the Elves sing of the end of our age, is it your name that brings the darkness or do you too cower in fear of another's shadow?
- Does the sun fill your wings as you rule your from kingdom high above the clouds or do you wear a crown fashioned from the bones of dead sailors, commanding the waves that crash on every shore?
- Did you you witness the first nights before any stars opened their eyes or do you still measure time in the passing of winter?