See the description of the project here.
I created two 'postcard sheets', each 8"x12", each of which I then cut into four 4"x6" postcards. I encourage you to make your own if you are so inclined, but if you don't have time, use mine! Here is a link where you can download all eight of the postcards. Just glue them to card stock, and send to:
President (for now) Trump
The White House
1600 Pennsylvania Avenue NW
Washington, DC 20500