I think one of the most useful skills one can have as a sandbox referee is the ability to pick and reskin other people's adventure modules. I mention this because it even if it seems second nature to some folks, especially those with a lot of experience with early play styles, I see lots of questions about it in "OSR" forums especially from people coming from newer systems.
First, to know what's good and more what's right for your setting. Not everything will be, not only are there a lot of bad or mediocre adventures out there, even some of the "classics", but there a ones that just won't fit. Having enough familiarity with your system and setting are helpful.
Second, knowing how to adjust prewritten stuff to make it fit. I find it's fairly easy, changing scenery and description - fiddling with monster stats. You can sometimes do it in play, but it's best to just write some notes over the thing. The difficulty becomes harder the more "story-based" a location is. That is to say if there's an entire narrative of hooks and connections or even more a series of events that occur in the adventure it's often harder to transform it into one's own setting then if it's simply a location to interact with. Even with the first though, the themes and antagonists in RPGs tend to be pretty simple and archetypical - which makes adjusting them possible much of the time.