For the donor: write one character in each box, the one printed in grey above it. Sit the letter on the darker blue line (that is the baseline) and let round letters like o and e reach the lighter blue line above it. Descenders on g, y, p go below the baseline as normal. Write at your natural speed — neat-but-normal, not your best calligraphy.
Use a black or blue pen. Not red, not green, not pencil. The blue guide lines are removed by reading the scan through its red channel, and a red pen would be removed along with them.
Do not touch the four black corner squares. They are how the scan gets straightened. If one is covered or cut off, the whole sheet has to be placed by hand.
Print all sheets at 100% scale (no "fit to page"), then scan at 300 dpi or higher, in colour. Colour matters: greyscale scanning destroys the red-channel trick.