It’s useful to consider what Doom has the potential to do, and this is the kind of thing that I’ve certainly learned the potential for over time. For me, it’s gone from being something that’s an interesting and useful element of a system I run, to being something I miss when I run games that lack something like it. That’s partly familiarity, and partly because I enjoy the way it gives me as the GM a way to engage with the game that’s more involved than the traditional “referee and narrator” dynamic.
So, based on some of my learned experiences, the following may provide some helpful advice:
Principles of Doom
In brief, Doom is an abstract representation of all the things that can go wrong during the course of an adventure. It’s unknown variables, unfolding conspiracies, malign influence, and the chaos of battle. It’s a discrete quantity of murphy’s law (“anything that can go wrong, will go wrong”), and the bullets in Chekov’s gun (if something is added to a story, it should pay off later - Doom is a way make those things pay off).
At the start of a scene, the GM describes what is currently going on - when and where it’s happening, who’s there, and what’s happening right now (and why it’s happening, but keep that secret). Once the scene begins and the PCs start doing stuff, the GM has two ways to influence the scene - NPC actions, and Doom.
Foreshadowing is a good idea here. When you use Doom, try to link it to the narrative; the easiest way to do this is to build on things established in the scene: if the PCs are crossing a perilous rope bridge, that justifies the GM spending Doom to make the bridge swing and sway unnervingly. The results of complication and failed tests, or NPC actions can also set up new things for you to spend Doom on. This ensures that Doom spends don’t feel like they’ve “come out of nowhere”, but it also means that players can actively try to prevent or shut down some uses: a cultist ringing an alarm bell justifies spending Doom to bring reinforcements, so preventing the cultist sounding the alarm can prevent those reinforcements.
Spotlight of Doom
Doom doesn’t just make things more difficult. You can use it to draw attention to things in the game too. Spending Doom is a sign that something is important because you’re putting resources into it.
Perhaps add a little extra roleplay or descriptive flourish to the NPCs you spend Doom on, like the camera in a movie lingering on a particular foe to show they’re important. You can do similar with the environment and terrain - the important places may change with Doom spent, as unstable cliffs crumble or ancient temples reveal hidden perils.
And it’s not just NPCs and places you can emphasise this way: a point or two of Doom can drop an obstacle in the way of the party that one of them is ideally-suited to overcome, shining a spotlight on what makes that character cool.
Obstacles of Doom
As with most GM advice about failure, the key to creating obstacles and problems with Doom is ensuring that they don’t make the game stop - an obstacle should make the game change direction, as players are encouraged to seek new solutions or find new paths forward. This is particularly useful if the players are dithering or overthinking things - spending a couple of Doom to make some bad guys burst in and attack can shake them out of their inactivity and get the game moving again (and maybe serve as an opportunity to drop in a clue).
Hopefully those musings on Doom come in handy.