White chalk marker 1-2 mm
To draw on your window, you'll need a chalk marker! These markers are suitable to draw on glass, mirrors and other smooth, non-porous surfaces*. In the marker is liquid water-based chalk, created opaque lines and is easy to remove with a wet cloth. Only suitable for children under supervision.
brand David Tutera
stroke 1-2 mm, bullet nib
Shake the marker (cap on!), then pump on a piece of scrap paper until you see the ink flow into the nib. Repeat this every now and again when you're drawing to get smooth lines.
If you buy a marker with a window drawing design, you will receive the marker in the mail and the printable via email. The window drawing designs are not physical products, but something you print it at home.
* Not ideal on chalkboard(paint) because that's too porous and you won't be able to completely remove it.