I made some pop-up cards from some kits that I'd bought, which I thought you might enjoy seeing. The actual principle is very simple, but until I had the kit in front of me, I didn't realise just how simple to replicate they would be.
Once you've folded your basic card shape, to the size you need to fit your envelope, then the fun begins. To get the effect of these cards, all that you need to do is to get your craft knife, or scissors, and make four cuts in the folded edge of your card. I usually take a piece of scrap paper from the recycling box to try out my ideas, rather than risk wasting my good card!!! That way I can play about with my ideas, and use the successful ones as templates.
Decide how far in to the card you want your first pop-out to be, then make two cuts, about an inch apart, it's really not necessary to be precise. Then repeat from the other side at the point you want your next pop-out to be. Open up the card, and pull the small projections you've just created to the inside of the card. You attach your topper to the front of these, and when the card is folded, they lay flat inside.
Alternatively, you might decide to have just one, single pop-out, or three - it's your card, you decide. You could have one central "statement" decoration, or off-set it and place a greeting tag to the side.
You might decide to have a small forest of Christmas trees, so make a few small projections for them to stand on, but make differing depths to your pairs of cuts, so that when the projections are in the card, they are at different positions, to give more perspective to the card.
If you have a large topper to support, you might find it useful to make a projection that is a couple of inches wide, or place it over two of the smaller projections, just to help it to stand, rather than flop!
Obviously, you can use the technique for cards at other times of the year, I'm just starting to think about Christmas!!
On the outside of the card, the cuts will be obvious to the recipeint of the greeting, and this would spoil the effect. To hide the cuts, simply cut some suitable backing paper deep enough to cover the incisions, both on the front and the back of the card, and stick it down. Then use a matching topper to the insert that you've made, to decorate the front of the card.
Voila - a miniature work of art to delight a friend or family member :-)