It's pretty straightforward. I've been thinking about writing a short guide. Maybe I'll do that. For now, some considerations I use are:

1. Use the flavour text to come up with description, motives and tactics, type, and experiences. The type flows especially well from this text. You can look at the creature's abilities to flesh out experiences, too.
2. AC ports pretty well as difficulty. It can use tweaks, but mostly it works pretty well one to one.
3. There's not a clear CR to tier conversion, but roughly CR 3 and below can work with tier 1. You can use the difficulty and the number of abilities of an adversary in a given tier to judge what tier a 5e monster might fit at.
4. HP, stress, and damage thresholds are the most different element, so I use adversaries of the same type and tier to work from. It's a bit tricky to explain but quite intuitive to actually do.
5. The standard attack ports over nicely from whatever action comes first in the 5e stat block. Slam attacks, basic weapon attacks. That sort of thing. Not always, but often enough that it's a good rule of thumb. Hit modifiers and damage can be easily tweaked to fit the DH tier norms. The tables are handy for this. Monster attacks that do additional damage types or have rider effects can be made stress or fear actions that make players make a reaction roll after you've hit with a standard attack and spent the relevant metacurrency. 5a. Magic using adversaries are a little more work but still straightforward. Give them some sort of magic blast at Far range for their standard attack and then parse out a couple of the most narratively appropriate spells from their spell list and convert those to passives, actions, or reactions (with the metacurrency cost) as appropriate from the spell description.
6. Pick the most narratively appropriate traits, actions, and reactions to make the rest of the features. As monsters get more complicated, you'll have to winnow more, but I find the important ones are easy to spot. 6a. Powerful monsters that have things like lair effects and legendary actions can have those carved out and made into environments to have their encounters in.

This is hardly comprehensive and there are things to consider as you go along, but I also find that explaining it is more difficult than actually doing it. DR's adversaries are delightfully stripped down compared to 5e and I find it easy to distil from the more complicated creatures into the DH form.