I sent this postcard earlier this year, I liked that it was a bit tech (showing how much I use my phone) and also a little bit old fashioned. So here is how I did it.
Step 1: Gallery
Start with the gallery, click the +file button for a new file.
Make the title something you want as the title on the top of the postcard.
Step 2: 2
The new folder is on the top left corner, drag the photos you want to it.
You have to decide whether you want to copy or move the photos.
Step 3: 3
I have chosen too many or too few photos here, when you turn the phone to landscape 6, 9 or 12 photos fit; 9 only if you leave the files on the side - by swiping the files to the left you have only the photos on the screen and fit 6 or 12.
I edited to 6.
Swipe the screen to save it to screenshots.
Step 4: 4
I used bluetooth to connect to the photo machine, this machine is a bit older than the one I used for my first one so I couldn't get even borders unless I cropped the edges which I didn't want to do.
This is the second card.