a piece of flat clear polystyrene of about 0.04in(1.0mm) thick
a suitably sized image of a phone screen
self adhesive white paper backing
a piece of clear acetate about 0.013in(0.3mm) thick
a piece of shiny plastic ribbon
a strip of self adhesive aluminum tape
Start with a small picture of a phone from a flyer (or print one yourself on thick glossy photo paper).
Note that flyers often show pictures with parts of the screen obscured by text or glare.
In real life most smart phone screens are somewhere between 3in and 6in diagonal so the screen on the image should be between 0.5in(12.7mm) and 1.0in(25.4mm) diagonal.
Find a source of flat unscratched clear polystyrene of about 0.04in(1.0mm) thick, e.g. a cd jewel case or some candy box.
The candy box in the picture is covered with a self adhesive label which can be peeled off, this label will protect the area behind it from getting scratched or damaged.
Instead of a label you can also use a piece of masking tape to protect the polystyrene surfaces from getting scratched.
Use a fine saw (I used an X-Acto mini blade in my #2 knife handle) to cut a rectangular piece of polystyrene similar in size to the phone on the picture, then cut some test pieces.
Better not use a knife as making cuts in clear polystyrene will cause tiny fractures to form around the edges of the cut which will end up being very visible.
Use the polystyrene test pieces and some unused parts of the flyer or phone image to test what glue will work (i.e. not damage the picture, stick to the polystyrene and dry perfectly clear).
The glue I ended up using is acid free white (PVA) paper glue.
Some of the test pieces can be seen in the picture between the candy box and the flyer.
Round off the edges of the rectangular polystyrene 'screen' using a fine file ('diamond' nail files are great for that).
Paint the edges on the inside of the polystyrene 'screen' black. Be sure to use a type of paint that will not get harmed by the glue used (e.g. latex based acrylics tend to get dissolved by PVA glue).
At first I tried to be clever by using a piece of masking tape to get straight edges only to find that the resulting raised edges of black paint caused the picture to lift from the screen, so I tried again this time painting the edges by hand (the edges ended up not perfectly straight, oh well).
I also painted the outside edges of the polystyrene screen black.
Once the paint has dried, glue the picture to the inside of the polystyrene screen, make sure not to trap any air bubbles between the picture and the clear plastic.
Let the glue cure, with PVA glue this might take a day or so since the water from the glue can only evaporate through the paper side.
After the glue has cured you need a backing of a uniform color behind the picture to prevent anything printed on the reverse side to show through.
The backing should also prevent the glue used for the rear of the phone from ruining the screen image (if you used an image printed on thick photo paper you can skip this step).
I used a piece of self adhesive paper from a set of postage stamps.
After applying the backing I painted a black dot near the left upper corner of the backing to simulate a camera lens.
I picked a piece of clear acetate-like plastic (clear, stiff, about 0.013in(0.3mm) thick), cut a rectangle of the same size as the polystyrene screen and rounded off the corners.
This piece of clear plastic is meant to provide a flat surface for the finishing layer on the back and a glossy lens for the camera on the rear.
I glued the plastic to the backing using an all purpose plastic glue (after verifying that the glue did not harm the backing or the picture behind it).
After a lot of trial and error I found a plastic ribbon with a shiny pearlescent effect that would not fray when cut and would not wrinkle when glued.
I punched a 0.08in(2.0mm) hole near the edge of a straight piece of ribbon, aligned the hole with the black camera dot on the rear and glued the ribbon to the back of the phone using an all purpose plastic glue.
After the glue had cured I trimmed the ribbon using a sharp hobby knife.
Using a template and a scriber I scratched the outline of a button below the screen (its position ended up a bit too low).
Finally I measured the total thickness and the circumference of the phone and cut a strip of self adhesive aluminum tape to fit.
The length of the aluminum strip is a few mm less than the total circumference to leave a gap at the lower edge where the connectors are located.