So, here are some tips on how to make a similar piece, either to display fabric postcards, or indeed, anything else you like - ATCs, maybe? or small journal quilts (I'm not sure how big a piece could be supported in this way, but I'm sure a 6"x6" mini journal could be - the larger ones, like the size of a piece of paper, might be a bit big for this technique, though it would be worth a try).
The joy of this wallhanging is that it can be whatever size you like, so you can make it to fit your space, which is what I did - I figured out how big a space I had, and worked backwards from there. I recommend sketching this part out on paper or in a quilt design programme. You can adjust the width of the sashing to suit your taste or your space (or both), or you could leave the sashing out entirely, though that will mean you have a few more layers of fabric to sew through, so you might not want to.
Essentially, the hanging is composed of a number of individual display modules. Each module is composed of one piece of background fabric, four squares to form the corner pieces, and whatever sashing you require/desire to surround it. The sashing could be added all around each module and then the modules joined together, or obviously, you could sash with short pieces on one side of your module, join several in a row and then sash all along the other side all at once - just like sashing any set of blocks.
My wallhanging has spaces for postcards in both orientations - portrait and landscape - but you could arrange yours any way you like. The advent calendar I got the idea from has 25 square pockets, so orientation is not an issue. Anyway, how many pockets you make, how you arrange them, how widely you sash them, and so on, is up to you, and I'm not going to address that here, but I'll walk you through making one display module - it's much simpler than you might think!
First, determine the size of the object you want to display. Postcards are usually 4"x6", so I went with that. There's nothing to stop you making a piece to display objects of different sizes, if you want to!
For each individual display module, you will want a piece of background fabric which is 1.5" larger in each direction than the object you want to display, so for the postcards, I cut the cream coloured background rectangles at 5.5"x7.5". This will give a finished size of one inch larger than the object in each direction, or 1/2" on each side - you could make it a little smaller, but then you run the risk of the pockets being too tight. Don't forget that fabric objects have a thickness that paper wouldn't have - with photos, for instance, you'd want a much snugger fit. I found that all the cards, even the ones that are a little on the small size, work well in these pockets - it's especially helpful for those that have funky yarn, etc around the edges, as they acquire a little extra thickness.
Then, you need four squares of fabric to form the corners. These work best if they are just under half the size of the length of the side of your background fabric (or in the case of a rectangle, the length of the shorter side). Make your life easy here and pick a number that's simple to rotary cut - if you have a 5.5"x7.5" rectangle, 2.5" squares are great. If you were making a 4" square display space (so you were working with 4.5" squares unfinished - I'd try 2" or 1.75"). What you are looking for is corners that won't overlap one another or cover too much of what you are displaying, but which will be big enough to securely hold the object you are displaying. You can always cut a few test squares and see which you like the look of best.
Once you have your four squares, fold them into triangles and pin them to the background piece, with the raw edges aligned to the edges of the background rectangle/square (see photo above). You can, if you wish, press the triangles before pinning them; I personally am too lazy for that - I simply pinned them and then sewed the sashing on.
Which is the next step - sew the sashing to the background piece, right sides together, keeping the triangles in place between the background fabric and the sashing fabric. (I have tried to show this in a photo, not sure it was really successful- the photo is the view of the edge you are going to sew, with the folded triangles stuck in between.) This is not as tricky as it might sound, though you may wish to re-pin with the sashing on, or just do what I did and feed carefully while pulling the pins out as you go.
And hey presto! The corner triangles are sewn in between the sashing and background, which keeps them in place. At the moment, they are only sewn down on one side, but the other side will get sewn down when you do the bottom sashing, whether you do it now or in a long strip later. (In this photo, the bottom right corner isn't really crooked, it's just flapping up a bit because I took the pins out.)
And that's really all there is to it. Once you've joined all your little modules up, how you quilt your wallhanging is up to you - I started to quilt mine in the sashing and decided the sashing was too narrow, so quilted in the background of each pocket. Which means that most of the quilting on my wallhanging won't actually show, but as it's utility quilting anyway, I don't mind. Just be sure NOT to quilt over the corner triangles, as they need to remain loose on the folded side so you can tuck your display objects into them...
Hope that helps!