Although it looks a little intimidating this is actually one of the easiest 3Ds to assemble. The best part is that this 3D once assemble, can actually fold flat and be sent in an envelope. You could write on the front or just store it as an ornament for Christmas or Valentine's.
First take the stucture pieces. These are the two pieces that are not hearts and look different.
Next fold down all the tabs on the top and bottom of each one. They are perforated to bend at the right places. You will notice there are two tabs in the center that are not perforated and do not need to be folded down.
Insert the folded structure piece into the slits on the largest heart. Be sure you insert the structure piece so that large heart rests next to the two tabs that did not fold. It should be in the exact center of the structure piece. Repeat for the second structure piece. After it is in place, fold up the tabs next to the heart to lock it in.
Take one of the middle sized hearts and insert it onto the structure pieces. Once in place straighten those tabs to lock it in place as shown above.
Take the smallest heart and place it onto the structure pieces. Note: if you plan to put a message on the front write it on the front piece before it is applied. Lock it in place with the tabs.
Now repeat for the back side. Apply the mid-sized heart and then the smallest heart.
Once all your pieces are in place and locked with the tabs. Push the small heart to the side to pop it up. Pretty cool!
Then if you want to fold it back flat, push it back and it will collapse.
Now mail it off to that person you adore!