Mark Landis, who traveled the country donating forged works of art to museums to please his dead parents, tells how he did it:

Frequently, Landis starts out with art catalogs or books. From these he selects art work that is small enough that it would be easy to transport, and that he thinks would please his parents — honoring his parents, who have both passed away, is an important part of the motivation he describes for donating copied pictures to museums.

“In 2000, I used to go to Office Depot or something like that and use their color copier, then, when I got the printer, I wouldn’t have to go any place,” Landis said. […] Landis pastes copies of a picture onto boards cut at Home Depot, then goes over a number of them in one sitting while watching TV. Using colored pencils, paint or markers, he fills in the image. To the naked eyes of museum staff, these works appear real when Landis presents them as gifts. But under ultraviolet light or a magnifying glass, the illusion falls apart.

How to Forge Art: Mark Landis Explains His Technique (Live Science)