So what do you need?
Find yourself a nice framed scene. By that, I mean a one piece cut out that consists of a scene welded to a frame, like this
Then, draw a dashed line through the centre, vertically. Make sure to release compound path and ungroup.
NB IF YOU DO NOT RELEASE THE COMPOUND PATH, THE KNIFE WILL OPEN PATHS ALL OVER THE PLACE!
Now make a copy of the outer edge and put to one side (incase you want it again). With the knife tool, cut the line on either outer edge, just where it starts to curve, in this case. Choose the vertical cut segments and make those lines dashed too. You end up with this
Well, almost this - I have offset the unbroken copied outline and centred it here and extended the centre fold line.
So this forms the inside of your card. You can follow the shape for the outer card like I did here
Or put it in a plain rectangular card shape.
So how did I get the blue background? The same outline shape was made ever so slightly interior offset and cut in blue paper. Use paper for this and card for the scene. The scene will stand up to wear and tear better that way and the paper will add less bulk.
Stick the paper insert to the inside of the card first, then the outer frame of the scene on top of that.
This is what it looks like on the front with just a simple phrase in gold pen and a couple of interior offsets. Use the outline shape and the knife on 'solid' to cut it in half to get the shape to offset.
See! Nothing to it!