I have referred to this a few times, with links out to pages on t’interweb where these things are spoken of. The drawback is that the web resources seem to use maths, when what we want is simplicity. So here’s my version.
What you need is a bit of card (I know, the title was a spoiler). Anything from credit card to an index card will do (ask your parents what a card index was). The main thing is that the card has one good right-angled corner. You also need a tape measure and a second scrap of card or paper.
Find a nice vertical line – a door frame or the edge of a wall. Measure a distance of say six feet from it and stand facing the target line with your toes at the distance mark. I say feet, but you might also measure the distance in meters. Use whatever units your camera lens is marked in.
Hold the card out in one hand at arm’s length. Choose the hand you will be using when you do this for real with a camera. If you are likely to be holding the camera in your right hand and using the rangefinder in your left, then hold it out in your left hand. Let’s assume you will be using your left hand. Close your left eye and line-up the edge of the card with the vertical target. Without moving the card, close your right eye and open the the left. Hopefully the vertical target will appear to move along the card, away from the edge that was originally lined-up. If it moves the wrong way, swap eyes.
That second scrap of paper is used to mark where the vertical edge appears to move to. Pinch it against the main rangefinder card and slide it sideways until your distant mark appears to consistently move to the same place. Keeping the rangefinder card and the scrap of paper pinched together, mark on the rangefinder card a line for that distance. In this example, six feet. Change the distance and repeat.
You can now throw away the scrap of paper and keep the rangefinder. You have built a rangefinder that works for you and your eyes and will measure close distances with enough accuracy to focus lenses using their marked distance scale.
Go ahead and make copies of the card so that you can keep one with each camera. Laminate them. Print the scale on the back of your business cards. Get the lines tattooed on your finger or mark them with a pen before you go out.
The only time you will need to change the card is if the length of your arm changes, or the distance between your eyes. Or you decide to become a pirate.