It's assumed that you're starting with something like Ben Eater's 6502 setup, which is basically the same as in Garth Wilson's tutorials, and the hardware specifics here are tailored for that. It should be easy to adapt for other arrangements though - so long as you have a 6522 VIA.
You'll also need a socket to plug your SD card into. I bought two of these simple microSD adapters from a popular online marketplace:
There's not much logic on the board itself, just a 3.3V regulator and an IC to level-shift between the SD card's 3.3V signals and your computer's 5V TTL-compatible signals.
I've also used a microSD socket that's mounted on the back of an LCD "shield" module designed for Arduinos - the details aren't critical and there are a lot of options out there.
Some adapter boards (though not mine) provide a signal to tell you whether a card is present, which you might find useful.
I'm not the first to do this, of course! 6502s and SD cards have been around for a long time and you're sure to find a lot of good examples if you search, especially on places like hackaday.
Wikipedia has a good reference for basic SPI communication: https://en.wikipedia.org/wiki/Serial_Peripheral_Interface
The following document was extremely useful regarding how SPI applies to SD cards, how to send commands, the initialization sequence, and what all the commands are: http://elm-chan.org/docs/mmc/mmc_e.html
You can also download the Physical Layer Simplified Specification from https://www.sdcard.org/downloads/pls/. I did, but didn't refer to it much.
As usual there are also many discussions on 6502.org.