Proxyfren's Great Whopping MakePlayingCards MTG Proxy Infodump
Q: What's a MakePlayingCards?
A: MakePlayingCards is a company that does exactly what they say - they will print custom-ordered playing cards of all shapes and sizes for a reasonable price, and more importantly, they'll let you slap MTG images on them as long as you remove copyright info or otherwise make it look like you're trying to straight-up print bootlegs. They make it pretty easy to custom order stuff, and also have options for different card stock, finishes, and even foiling. Depending on the size of your order, the price per card is around $.30-20 USD.
Q: Do the cards look and feel alright?
A: They're exactly the same size, have a pretty similar texture, rarely have centering issues, and can look awesome provided you send them quality images to print. The minimum size for card images is 300 DPI, and it's better if you can get larger.
Q: So can I just drop card scans into my order or what?
A: No, unfortunately. Because of how the sheet cutting process works, each image is required to be a bit larger than what you actually want printed out. Thus, if you wanted to use a hi-res scan, you would have to add an extra border around it. Just look at any MPC-formatted image and you'll get the idea; the black border around a (non-borderless) card will look extra chunky.
Q. How do I get started?
A. See below.
https://www.makeplayingcards.com/design/custom-blank-card.html - This is where you start. The default options are perfectly fine (S30, full color, MPC game card finish, shrink-wrapped). The more cards in your deck, the cheaper the cost-per-card. NOTE: Shipping not included in the estimates on this page. Usually ranges from $10-20 depending on the size.
http://mpcautofill.com/ - A wonderful tool. Input a list of cards, select images, download a generated XML, and run the program. You're probably gonna use this.
https://www.reddit.com/r/mpcproxies/ - Most discussion on MPC-formatted proxies is posted here, and there may be some images here that autofill doesn't have.
https://mega.nz/folder/XWo2AQYZ#Y4S6_xZ1OqdDKKTMPuUjUw - My crap. LardFetcher memes, Hell's Cube, custom cards, and some alt arts, all shoved into a template and with copyright info removed.
http://waifu2x.udp.jp/ - AI Image upscaler. You'll likely want this if you're handling cards made with Magic Set Editor, which outputs at 150 DPI. I personally use Artwork, Low, 2x for MSE cards, which spits out an almost perfectly sized image. There are other sites out there that do AI upscaling, I just use this out of habit. Text can upscale poorly and turn out watery, but it's passable at bare minimum when printed out. Would recommend pasting high-res art over the upscaled card's art box, since it isn't much trouble and looks better.
https://www.reddit.com/r/bootlegmtg/comments/7vgq9w/another_step_by_step_guide_for_mpccom/ - A guide with a (really basic) template.
https://www.gimp.org/ - The free image editor. Just open the template above in GIMP, drag n drop your unformatted images, scale that layer to an appropriate resolution (usually ~745x1040), and remove any copyright info.
https://www.reddit.com/r/MTGLardFetcher/ - A source of meme cards.
https://www.reddit.com/r/HellsCube/ - A source of technically-playable meme cards.
https://www.reddit.com/r/custommagic/ - A source of supposedly playable, mostly non-meme cards.
https://magicseteditor.boards.net/page/downloads - Magic Set Editor, for Windows. Download the Advanced or Mainframe version. (There exist hi-res, preformatted MSE templates, but the ones I've found only support a limited selection of card types/borders, and have text-scaling issues. Wouldn't recommend those.)
https://mtg.design/ - An online card creator. MSE has more variety, but this spits out 300 DPI images.
If you just want to print proxies of real, existing cards, use mpcautofill and/or trawl /r/mpcproxies for images.
If you want to make your own proxies of custom cards or whatever, grab the card images you want, upscale them with waifu2x if needed, drop them in the template, scale the card layer down to an appropriate resolution (746x1039 is roughly 300DPI), and remove any copyright info. Would recommend pasting high-res art (if available) over an upscaled card's art box when possible.
You can sometimes get away with a few cards in your MPC order having copyright info (you might do this if removing the copyright info isn't a clean job), but it's a crapshoot depending on what employee reviews your order. Your mileage may vary.
You can print stuff on the back of cards too! If you're printing out dumb, non-functional memes, maybe consider a basic land (or anything) on the back.