Plan the card
Choose the finished size before you make the image. A2 cards are 4.25 by 5.5 inches. A6 cards are 4.5 by 6.25 inches. A 5 by 7 inch card is common for invitations and holiday cards. Square cards work well for a simple centered subject.
Match the prompt's framing to the card. Portrait cards suit a tree, hat, or character. A square format suits a centered icon. Keep the focal point away from the fold and the outer edge.
Leave room to write
Each source prompt asks for a large bordered blank area for writing and a defined border. Keep that direction when you want to add a handwritten message. Ask for empty space in the upper third, lower third, or center. Then check the result before you print.
Do not place a face, text-like marks, or small details where the message will go. A plain field is easier to write on than a busy pattern.
Set up for print
Keep important parts at least 0.25 inches inside the trim edge. If a color or image should reach the edge, extend it about 0.125 inches beyond the trim line for bleed. Keep the border inside the safe area so a small trim shift does not cut it off.
Print a test at the final size. Check the fold, border, ink coverage, and the blank area. Use a heavier uncoated card stock if you want a pen to write cleanly on the inside.
Prompt examples
Related walls
Published here first. This one never made it to Medium.