All you need is an idea and a decent camera with a timer on it.
Now, full disclosure, I have a DSLR camera but it's quite a few years old, and I use PhotoShop. However, with the cameras in most phones these days, for the size your card will be, it's not necessary.
Just setup some sort of a scene, like we chose Xmas socks by the fire.
If you have an iPhone for example, portrait mode can replicate this pic I took with my DSLR, the desired look is your subject (our feet) in focus and the fireplace blurred. Again, portrait mode will do this.
You'll need something to hold your phone or camera, I used a tripod. Could be anything that'll hold the phone/camera still.
Once you get your phone/camera in place, get your peeps in place, tap the screen or tap the exposure button to get your subject in focus, then you want to utilize the timer on said phone/camera, assuming you, too, want to be in the picture
Will likely require multiple takes and some practice, but as you'll see below, good results are totally possible. We're pretty happy with our Xmas cards ordered through Costco.
Total cost: $34.99 for their premium stock cards.