Part 3 - Printing
Level 0: Not printing
So I have to plug this in here. If you don't want to print anything, e.g. you're just going to use these in Tabletop Simulator, then you can stop reading right here. Furthermore, the Custom Deck Builder is probably better to use than Photoshop, so you wasted you time on that section. Maybe this is a bit out-of-order with that in mind. Whoops, lol.
Level 1: A regular printer
Whether you own a printer of your own, or you have a Staples or Office Depot nearby, the vast majority of cards you print should just be on plain printer paper from a plain printer, color recommended but optional. Like I mentioned in the concept phase, you'll want to re-work your cards and often, so you don't want to spend a bunch of time and money on nice prints that are just going to go in the trash.
Print your cards on plain paper (optionally using my Regular and Oversized templates to make arranging cards on a page easier), cut them out with scissors or a guillotine, then throw them in front of another card in a sleeve. If you have any thin cards lying around, like a deck of cheap poker cards, feel free to use those to keep the feel of the cards similar to the actual cards, but the goal for this step is to get these in a state where you're satisfied and ready to move on to the next level. Having these be a bit thicker in the interim while you're working that all out won't hurt your hand, but doing the inverse will definitely hurt your wallet (ask me how I know).
Level 1.5: DIY
So I've seen some half-completed resources around the web about making your own playing cards without moving fully to Level 2. If you've gotten this far, you've probably seen them all, too. I've explored these options and have found them to either lead to a dead end or yield lower quality results than what I'd be satisfied with. As such, I don't use any of these methods, but I'll list them out for posterity:
I'm not gonna say the above options won't work for you, but they don't work for me, so this is all you'll get here.
Level 2: Professional printing
This is the end game. You've designed and re-designed your cards and you're ready to commit them to paper for the last time. That means it's time for you to shell out for a professional. I know, kinda anti-climactic for this whole DIY endeavor to end with spending money for someone to do the rest for you, but it's just what I've found to produce the best results with the least headaches. You're free to cut things off before this step or explore the above options if you want, but since you've gotten here, you might as well stick it through to the end.
There are a bunch of different printers, but I have personal experience with and would recommend three (really two, but I'll explain that later). For all of them, here's preliminary information that you need to know:
With that out of the way, here are the professional printers I can recommend:
If you've seen my custom card posts, you'll probably noticed that they all seem to have a lot of extra border. This is because when you get cards printed professionally, you need to have that extra space to account for physical variations in the print process. While these tend to be minor, it's the best practice to over-compensate for them (and the printers kind of force you to anyways). And don't fret it, I've never seen more than a millimeter of variance on any of my actual prints. There are three boundaries to be aware of:
Any Photoshop template you get from me has all these above areas built in with a solid red line, dotted blue line, and solid green fill, respectively. This is the key area I feel the Custom Deck Builder lacks, as it does not give you the extra space to account for the physical print, which can lead to either tragedy or a bunch of extra legwork, both of which defeat the purpose of using the tool.
MPC (and Printer Studio) have been progressively cracking down on what they feel are copyright issues, namely anything that their auditors can positively identify as belonging to major IPs (which includes DC, Marvel, Star Wars, and I'm sure countless others). If they identify anything like that in your order, they will ask you for proof of permission from the rights holder and then ultimately cancel your order. They will fully refund you, but it seems like once you get blocked once, they hard check everything from you forever.
I still recommend MPC as the first place you go for custom prints. If you are going to use materials that could be construed as belonging to these IPs, I've had a lot of success in the past by front-and-rear-loading the less identifiable stuff and sticking the more identifiable characters (e.g. Batman), since it seems like their auditors mostly spot check the beginning of orders. Your mileage may vary. Print & Play does not care at all about what you have them print, as far as I've seen.