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—
"Okay, that's out of the way, then, next up is—" Pause. "Eating and sleeping, actually, I ought to go do that, it's getting fairly late for my bio clock even though I'm still pumped."
"Do you want to have some fairy food or go back to the mortal world?"
"Back to the mortal world, and might be a good time to register your fingerprints, too."
"Safety inactive," she calls when they're in, and she leads Promise downstairs to one of the computers, opens a certain program, and offers her a hand scanner. "Put your hand here, please."
"Alright, you're registered. Tomorrow I'll show you the places where the scanner's hidden in the other four houses, but you're good here for now."
"Oh, and also I got places here for you and Yellow to sleep if you want or need, though I expect you to want to sleep in your tree."
"I want to sleep in my tree. By all means take Yellow back with you."
Up she leads Promise, showing her the appropriate place to scan her palm and deactivate the security system again, and then back to fairyland to fetch Yellow.
"Want anything to eat in particular?"
So she fetches two sandwiches for herself to eat and a couple of bananas for Yellow and goes feed him while she reviews some stuff on her computer.
Right now she's looking at delivery times for various pieces of equipment and sending someone an email (using Tor) clarifying some things about the design of something.
"Careful not to break anything," she says, not taking her eyes off the screen.
"I know, just in general. It shouldn't be easily breakable, anyway, and everything here's redundant ten times over, but." Shrug.
And after only a few more minutes Mortal announces she's going to brush her teeth, take a shower, and retire to bed.