Step 3: Using a level shifter
Just connect power and ground, and the four signal pins.
It's only a matter of figuring out which four (digital) Arduino pins
to connect to, right? Well, yes, but there's a complication.
SD cards, like many recent electronic devices, operate on a 3.3 volt logic
level. But, most Arduinos output 5 volts on their digital logic outputs for ON.
A direct connection of 5v to an SD card could fry it. So, what do we do?
The best solution is something called a level shifter. Essentially, this means
putting 5v on the input of a non-inverting buffer and getting 3.3v from the output.
Shifting the voltage level, get it?
But just how is this magic accomplished?
One way is to use a CMOS hex buffer IC,
here a 4050, and power it (pin 1, Vdd) at 3.3v.
Put 5v on the input of one of the gates,
and you get 3.3v on the output of that particular
buffer. For example, with 3.3 v on pin 1 (Vdd) and
pin 8 connected to ground, a 5v logic level on
input pin 14 (gate 6) results in 3.3v on output pin 15.