Renting a flat, opening a bank account, a notary appointment: someone wants both sides of your ID, on one page, on paper.
The obvious method and its problem
You photograph the front, photograph the back, and put both pictures in a document. It takes a minute and it usually produces something wrong.
The card comes out too big, or too small, or one face is noticeably larger than the other because you stood slightly closer for the second photograph. Sometimes it is stretched, because the photo was cropped to an aspect ratio the card does not have. It is legible, and it is obviously a snapshot of a card rather than a copy of one.
Most of the time nobody objects. Occasionally somebody does, and you are back at the photocopier.
85.6 by 54 millimetres
There is a standard, and your card follows it. ID-1, defined by ISO/IEC 7810, specifies 85.60 by 53.98 millimetres — the same footprint as every bank card in your wallet, which is why they all stack so neatly.
Knowing the number changes the problem entirely. You are no longer trying to make a photo look about right on a page; you are placing a rectangle of known physical size onto a sheet of known physical size. A4 is 210 by 297 millimetres. Two cards at true size, stacked with a margin between them, fit comfortably with room to spare.
Recto composes the page that way: it places each face into an 85.6 by 54 mm frame at the correct position on A4, fitting your photograph inside without distorting it. Print at 100% — not “fit to page”, which will quietly rescale everything — and hold a ruler against the result. It measures.
Photographing the card itself
Composition can only work with what you give it. Flat surface, card square to the camera rather than at an angle, and light coming from the side rather than over your shoulder, so the flash does not bounce off the laminate straight back into the lens.
Fill the frame with the card. Cropping later throws away pixels; getting close keeps them. If your phone offers document scanning, using it is better still, because iOS will find the edges and correct the perspective before you ever see the image.
Where the copy goes afterwards
This is the part worth pausing on. An ID copy is among the most sensitive documents you will ever make, and the usual workflow involves handing it to a free web service in exchange for a layout.
Composing the page on your phone avoids the question entirely. The photographs stay in your camera roll, the composed page stays in the app until you decide where it goes, and no third party ever had a copy to delete.