Assemble the circles.
Slide together the slots on pieces A and B, and on pieces C and D, as shown.
Flip them over to the back to check that they look like this. Flip back to the front side.
Create your designs.
On my pumpkin the A-B circle is the one you see in the "start" position. The A-B tabs start at the top, are pulled down counterclockwise, and end at the bottom. The C-D circle is visible in the "end" position. Depending on your design, you may want to start with the tabs at the bottom. In that case you would see the C-D circle first. The A-B tabs would be pulled up, clockwise, and the A-B circle would be revealed at the end.
Place A-B circle so that the tabs are at the top. Place C-D circle so that the tabs are on the right. Draw, stamp or glue your designs on the circles. Do not glue anything across the center slots. The mechanism will not work if the slots cannot open. (On my pumpkin the mouth is cut and glued as two separate pieces.)
Take the circles apart.
Lay A on top of D, aligning slots. Lay B on top of C, aligning slots.
Treating the A-D sandwich as one piece, and the B-C sandwich as one piece, slot together the two parts, just as you did with the A-B and C-D circles.
When the pieces have been slid completely together, tab B should be exactly on top of tab A, and tab D exactly on top of tab C.
Hold the C-D tabs together in your right hand. Holding the A-B tabs together, pull down to make sure the circles turn properly.
Lift tab B and spread a little bit of glue on tab A, gluing tabs A and B together. Likewise, lift tab D and glue it to tab C.
If you make one, send me a picture and I'll add it to this page. I'm sure your designs will be better than my extremely artistic sun and moon. :)
Here's one by MariLynn.
Check out another way to make a changing circle card at Addicted to Stamping.
If you're looking for a rectangular changing picture card (also called a cross fade card), there are instructions and templates in both of these books.
It's on page 117 of Rob Ives' book, which you can preview on Google books.pop up cards papercrafts