For this I could use a USB microscope, but the price is higher and they are usually too big. A normal camera can not focus closer than about 10 cm.
So I need a lense of some sort, preferably a cheap one. The cheapest one I found in my house was a loupe I use to look at SMD soldering. It is made to examine stamps, jewelry and other small things and costs just 20kr (ca. $2). Just mount it in front of the camera and suddenly the focus is down to 3-5 cm. Instant macro! :)
Step 1: Buy a Cheap Microscope or Loupe
Step 2: Tape the Camera to the Lens of the Loupe
Place camera lens so it is about centered over the loupe lens. The position does not need to be precise.
Step 3: Fold Loupe Around So It Fits Your Camera
With another type of camera there may not be space to fold the case around. Unnecessary plastic bits can always be cut away.