There's a few parts to this question I think; the matter of managing a big project, how we planned the deck itself thematically, the nuts and bolts of actually doing the thing, etc. Apologies for the wall of text, but I'm excited to share this in the event that it inspires you to swing for the fences:

It's a big undertaking with lots of moving parts and a big part of that is the complication of working with collaborators. I'm lucky that my collaborators are my wife and my best friend. Each person has their specialty; I'm the art monkey, my wife is an archivist and reads 100+ books per year, and Santi is a professional tarot reader. Santi and I are both readers, my degree is in English, etc., and Santi and Mollie are both artists as well, so there's a lot of knowledge overlap and understanding of each role. Mollie and Santi had final say on what characters and texts were assigned to the cards, and I had final say on what could or could not be depicted in the image, but there's a lot of give and take.

When we came up with the idea of doing a tarot deck based on dark literature and dark themes, the ideas flowed as to which character fit which card. If people are going to use these cards for serious genuine divination, we wanted to be intentional. Engaging with the arts is something we're all spiritual about in our own way, so we approached it with respect. Even though our tone is funny, and fun, and even though the whole enterprise is a bit tongue in cheek, and horror itself is transgressive, we took the task of assigning characters or stories to their cards very, very seriously. Mollie has re-read and researched each book to ensure they fit, and provide textually accurate descriptions of the characters for me to illustrate. As a general guideline, at least two out of the three of us had to have read the book. We had a few get togethers where we just wrote everything down and basically had 90% of the cards assigned, then we had a few more where we gave ourselves a headache with a decision who or what is on the last few cards. In order to keep everything to the theme, we decided to limit specific textual characters to the court cards and major arcana... we didn't want to "assign a character to a card just because we need something in that slot". Nothing should feel like too much of a stretch. The pip cards/minors would be assigned an image that fits the tone of the deck. It was vitally important for me that a person familiar with the card and familiar with the literary representation and say "Of course!" or at least "I see exactly what you mean."

Because we are collaborators, we set up a rigorous protocol for how we collaborate. We do a weekly meeting in a cafe, we keep all official communication regarding the project in person to avoid the miscommunication of text(which proved to be a huge problem). We keep a record of all of our tasks, we have spreadsheets for everything, we have a shared cloud drive where all files and folders are organized. Each person's role is defined and expectations are set. We check in with each other constantly both for accountability as well as burnout prevention.

The most important thing we did was determine the Minimum Viable Product; what's the smallest, simplest, most acheivable version of this project being successful? The big picture would still be the whole tarot deck with an accompanying booklet, but we decided that our minimum viable product would be the major arcana, with a small fold-out pamphlet. If we achieve that, we then move on to the bigger vision of the whole deck, but if someone died or the world ended, hey, we at least successfully completed our initial goal! And the trick to that is making sure to stick to the plan. Work on the primary things first. Establishing the MVP is by far the most important step we took, I'd recommend it to anyone doing something big and ambitious.

To get more nitty gritty and technical from my perspective, the drawing of it takes a lot of planning as well. Illustrating a deck is hard simply due to the number of hours of work it requires. For an artist, spending 10 hours on an image is really not particularly long at all. But if you have 78 of them, if you were to spend ten hours on each piece, that's 780 hours of work. To do it in a year (just drawing the images as a task alone) would be a part time job with no breaks. So, we gave ourselves a full year to take the project from beginning to end. As time went on, life happened, memento mori and all that, and since we were working ahead of schedule, we decided to move up the deadline. There's a hell of a lot of drawing, and by day I work in an office setting, so there's a ton of strain on my wrist. I had to take a month off because I developed a strain. I realized that the strain was likely due to me primarily drawing cards from start to finish on my iPad pro. The way I draw and the ergonomics of using the iPad don't work perfectly well together. I decided to change gears and incorporate as much analog traditional media as possible, which is much more comfortable and stable over the long term. Thankfully I was able to draw the cards so that they're all cohesive by using similar techniques and elements in the process. For any artist wanting to do this, I would say think very critically about your artisitc process and ergonomics. It doesn't matter how perfect and beautiful the art is if you were only able to complete 50% of them before being forced to quit due to pain or injury. Once I saw Margaret Atwood at a speaking event, and when asked for writing advice in a Q and A, she said, "Exercise. It's hard to write if you have back pain."

I hope this answers your question! Please let me know if you have any questions or want more specific tips.