The plan is refined and strengthened, contingencies are set to deal with various ways Thorn could've strengthened his defences, Mortal receives a very interesting email concerning one of their mother's contingency plans—namely that her assets have all been either frozen or transferred to Mortal themself, so Thorn doesn't have access to them -, and when Mortal and Promise judge there's nothing more to be gained from further planning they (eat dinner, sleep again, have breakfast, go over the plan once more when it's not completely fresh in their minds so they see if they come up with anything new, purchase Promise a mobile phone and a power generator to charge its battery in fairyland, eat lunch while Mortal teaches Promise how to use that, and) part ways.
On this side of the veil, Mortal gets to work. The first step: getting at least two safe houses, one for their HQ and the other for contact with the place near Thorn's court. They end up getting five, for redundancy's sake. The main HQ is near Seattle, the other four safe houses are in Greece, Russia, Japan, and Argentina. That, and getting the necessary existing equipment, is the easy part—you end up with contacts of the relevant sort when the bulk of your wealth comes from the kind of thing you can get with sorcery. The hard part is getting the various specific bits of technology that don't exist yet, including the the many types of trap and ammunition they'll need. Processing fairy voices with software turns out to be a dead end—apparently they're weird magical superpositions of sounds that make software go ?!?!?!?!?—but everything else, as agreed upon, can be made to spec nicely enough.
It'll take a couple of weeks beyond the one month for everything to be ready. Mortal hires someone who talks very fast.
And after the ball's going, there's not actually much for them to do with their time. They fret about details of the plan, order more redundant pieces of stuff (especially the to-spec stuff, not being mass-produced means they'd better have a lot of it to start with in case anything goes wrong), and have various antsy and anxious and calm and relaxed and terrified and panicked moods. A month is a long time...
At one point it occurs to Mortal that Promise might've decided to run away and not help, and then they'd never see her again and that would be terrible, and why would it be terrible anyway? It's just some fairy, fairies are evil, one must remember that. Even though she wasn't, of course, she was smart and resourceful and moral and ridiculously hot, and if they never see her again she'll never order them again and the tingly feelings won't ever happen again. Except what the heck, what are they even thinking? The answer, of course, is that they want to see Promise again. Why? To save their mother, of course. The only reason being ordered like that felt good was because Science. Of course. Of course.
The month passes—
"So around three and five hundred miles respectively... Yeah probably be best to open at least a small gate near each site."
"That depends on a lot of factors including terrain details but with anything less than a fifteen-feet antenna we won't be able to get much more than about fifty to a hundred miles. Might be ideal to ask our first vassal to open very small gates tucked away somewhere just for signal propagation."
"They won't all know how to do gates, it's an obscure skill even among advanced sorcerers - it's not that hard but there's so seldom a reason to do it."
"Hmm, yeah, then it might be best for us to create one gate near each site, for the aforementioned value of 'near.'"
"Is, erm, fifty miles too close? Is he likely to have someone patrolling there, or...?"
"So that should probably be enough, then. You can go the longest and safest way, so you don't get closer than that at any point."
"A radar ought to be able to detect the structures at more than that distance, you could be informed of where it is long before you reach that radius."
"It's, er, yours will need to be slightly bigger than this, these are all cheap short-range versions, but here," he shows her it, "an active radar has a screen like this and as you move it'll show you relevant terrain features within range. Artificial structures ought to stand out."
"Okay. He prefers buildings to being dug in underground, so that should do it."
"Yeah, dug in underground... I suppose I'd be able to get a seismograph that could detect it? But that might take a bit longer."
"Well, he's not an underground sort of fairy so his court sites should all have buildings and there shouldn't be anybody else that close."
"Should be fairly easy, then. I'll also be keeping an eye and ear on you?"
"Yeah, camera and mic. You might want to keep a dart gun with you in case you need it."
"That's probably a good idea. I can juice some haws - mix it with some mortal food of yours in case it's somebody from Thorn's court who has my name."