The most important part of planning is understanding that the plan will fail upon contact with the enemy. The second most important part is to have fun and be yourself. You're intending on striking first and striking quickly; if you can recruit these thieves, all the better, but they need off the table.
If you recruit them, you're going to need some way to keep them paid or at least fed and provided for, and to consider if the risk of bringing the local law down upon you is worth it should you have them keep stealing. Manual labor might be a possibility for them but that's a problem to pick up when you know them a bit better.
If you even choose to leave them alive, anyway.
You know one entrance to their hideout, and there is likely another. You sketch out a map of the local area and try to think if the city has sewers, but you're pretty sure it doesn't; carving them into the solid fucking rock of the plateau would be a problem, which doesn't seem that way on the face of it but you've seen a monsoon season or thousand in your time and the last thing anyone wants is the sewers flooding when your society doesn't own a single square inch of honest soil to absorb water with. So the basement can't connect to a sewer system. Maybe an emergency tunnel? You tap a few buildings that Jesca owns with the end of your pen, and you think...
A note: words like 'eliminate' in these contexts mean to take out an asset or enemy such that it cannot influence the mission; it is not necessarily lethal, though it can be.
Imps
The little bastards are poisonous, turn invisible, and can shapeshift into animals. You have a LOT of them. They can take
one action in the planning phase.
[ ] Watch the perimeter and prevent escape
[ ] Lurk around near you as your bodyguards
[ ] Try to find the back entrance by dint of sheer numbers and mobility
[ ] Infiltrate ahead of the Operation and bring back intelligence
Agents
You're honestly not sure what they're capable of but it's gotta be something. They can take
one action in the planning phase.
[ ] Watch the perimeter and track any runners
[ ] Go with you as your bodyguards
[ ] Prepare to report the thieves to the law on your signal
[ ] Get rubbish wagons around for convenient corpse disposal
You
Your best friend and worst enemy. You can take
two actions in the planning phase.
[ ] Attempt devil summoning and call [NAME CHOICE HERE]
[ ] Find the rear entrance yourself
[ ] Stake out the hideout and eliminate thieves when they go for a job
[ ] Prepare a Contingency (2 Supply; non-action)
[ ] Enhance your agents and imps with basic necromancy (temporary)
[ ] Research relevant local laws so you have better threats to negotiate with
Approach
First impressions are important
[ ] Stealthy
[ ] Open & diplomatic
[ ] Seize by force
&
[ ] Commit to the "main" entrance
[ ] Take the rear entrance if it can be found
Thank you for your patience
Voting will be in plan format
Victory, as always, to the Lord Below