Upload a PDF or photo, mark the Front and Back of the card, and print card-size copies on A4.
Drag on the page to draw the Front box (roughly is fine, it snaps to the card). Drag a box to move it, pull any corner dot to resize.
Several cards (for example two Aadhaar cards) are placed on the A4 sheet automatically, row by row, and continue on a new page when it is full.
In the print dialog, set Scale to 100% / Actual size. If printing is tricky, download the A4 PDF and print that.