The slide mechanism is not my own development. I saw a YouTube video with the information and adapted it to create a 3D card with a secret message. It's really easy, and kinda fun!
Here are the components you will need to make a 5 1/2 x 4 1/4 card. These pieces / measurements can always be finagled to suit your own needs.
A. Standard 5 1/2 x 4 1/4 card.
(Not Pictured: Cut 2 scenery pieces: One 5 1/2 x 4 1/4 piece, and the second piece should be 5 1/2 x 2 1/4.)
B. Message Strip: 3/4 x 7
C. Slide Track: 5 1/4 x 1 1/4
(Score this piece lengthwise at 1/4 and 1 inch.)
D. 2 Slide Tabs: One tab should be 1 1/2 x 3/4, and the second tab should be 1 x 1 1/2 inch.
E. 1 Pull Tab Decoration
F. 1 Slide Decoration
G. 1 Brad
(Not Pictured: Choose a sentiment for the secret message).
Step 1: Adhere the 5/12 x 4 1/4 scenery piece to the card base.
Step 2: Adhere the pull tab decoration to the left end of the message strip, and stamp the sentiment on the strip about 1/4 " from the pull tab.
Step 3: Just above center of the slide track, cut a 3/16 x 4 3/4 window.
Step 4: Fold and burnish the scored lines.
Step 5: Fit the message strip inside the track until the pull tab is flush against the right side of the track, and none of the secret message shows. Trim the left end of the message strip to the length of the slide.
Step 6: Line up the 2 slide tabs, lengthwise, at the left end of the slide with the message strip inside. Pierce all 3 layers (2 slide tabs and the message strip) through the window of the slide track.
Step 7: Remove the message strip and slide tabs. Adhere the slide track to the card base about 3/4 down by taping the top and bottom folded edges only. Do not tape down the right and left sides of the slide track.
Step 8: Run the brad through the 1 1/2 x 3/4 slide tab and the back side of the message strip so that the "legs" of the brad are pointing up.
Step 9: Bend the brad's legs flush against the message strip and insert it into the slide track until the pull tab is flush against the slide track. This may require some gentle finessing to get the brad's head into the track.
Step 10: Unbend the brad's legs. Insert the brad into the 1 x 1 1/2 slide tab, lengthwise where it is pierced.
Step 11: Bend the brad's legs so that there is a little gap. This may require tweezers.
The gap will allow the slide to function.smoothly, while bending the legs flush will stop the slide mechanism from working.
Step 12: Using dimensionals, adhere the slide decoration onto the 1 x 1 1/2 tab just above the slide track.
<--- See how the brad looks?
Step 13: Tape the 5 1/2 x 2/14 scenery piece to the bottom of the card base. Tape only the bottom and 1/2 of the sides under the slide track. Do not tape the top portion to the card base.
Step 14: Grab the pull tab and slide it to the left to watch the slide decoration move across the card and reveal the secret message!
Thicker card stock may require the slide track to be scored twice on the top and bottom to create a "box" shape. The slide track should be cut at 5 1/4 x 1 1/2, and scored at 1/4, 3/8, 1 1/8, and 1 1/4. Fold each score line, and adhere the top and bottom folded edges to the card base.
The pull tab and slide decoration should be of sturdy material. GKD's wood embellishments, and stamped card stock treated with Glossy Accents are a couple good choices for these pieces.