As a detailed description

Think about the hazard and its specific fictional details: Where is it? What does it look like? How does it work? What triggers it? What does it do? You might just visualize it in your mind, or jot some notes down, or even draw a picture or diagram.

In play, introduce this kind of hazard much like you would if you were improvising it. Frame a scene where the hazard is relevant, introduce it with a GM move, ask "what do you do?" Answer the players' questions and resolve their actions based on your understanding of the hazard.

This approach tends to be good for traps, because it means taking time in advance to think through how the trap works. When you improvise a trap, it's easy to make up details that don't make sense under scrutiny.

This approach is also good for physical obstacles, especially ones that aren't particularly dynamic (like a chasm they might want to cross, or a tree they might want to climb). Having thought through the physical space allows you to easily answer the players' questions and resolve their actions.

For our first adventure, there's a ruined crinwin nest that the PCs might encounter en route to Sajra the swyn's lair.

I suspect that the PCs might want to investigate the nest, or set fire to it, but I know that it's high up in a huge tree. The climb would definitely be a hazard.

I make some notes: the nest is about 70 feet up, and the lowest boughs are maybe 40 feet. The trunk is maybe 10 feet wide. I look up sequoia redwood trees and make a little sketch for reference.

A fall while climbing a tree this tall could definitely kill a person, but it's not certain death. It's definitely big and scary, would knock someone around, and armor wouldn't help against it, so I set the damage as 1d10+2 (ignores armor, forceful*).*

There's also an old Maker greenhouse near Sajra's lair, which enslaved crinwin have been excavating for the swyn. I decided earlier that it would include some old magical seeds, locked away and still unpilfered by the crinwin. Why? Because they're protected by a trap!

I think that the seeds are stored in an impossibly well-preserved wooden chest. If anyone disturbs the clasp without first pressing two particular knots in the wood, a thorn will shoot out of the clasp and prick whoever is handling it. The thorn carries a lethal poison (d10 damage, ignores armor, will quickly necrotize flesh and stop a heart if not treated). There's a dead crinwin in the room with a withered black hand; it tried to open the chest earlier.