Open a new canvas, 400 x 400 pixels (or larger, if you want a larger frame) and with your Preset tool set to rectangle, draw out a rectangle the size your want (foreground closed, background set to black or colour or pattern of choice). Objects – Align – Centre in Canvas. Convert to Raster. Change your background colour to white, and with your Preset tool set to rounded rectangle, draw out a rectangle inside the black one. Objects – Align – Center in Canvas. Convert to Raster.
Click inside the white square with your Magic Wand (Tolerance set to 0), then Selections – Modify – Expand, and expand by 2 pixels. Go to the layer with the black square and hit delete. You can also delete the layer with the white square. Select none. This will be your basic film frame.
Now comes the fun part! Add a new layer and choose another preset shape or ding of choice. Set your background colour to any colour that shows up well against black (foreground colour closed off) and add to your frame. Simple shapes work best, but you can also experiment with other shapes. Duplicate your shape as many times as necessary and arrange to your liking. Close off your black frame frame layer and merge the other layers (Layers – Merge – Visible). Open your black frame layer again.
Duplicate your pattern layer, then Image – Flip and move into position. Close off the black frame layer, then Layers – Merge visible. Open your black frame layer.
Go to your pattern layer. Selections – Select all – Float – Defloat. Go to your black frame layer and hit delete, then delete your pattern layern. You can now duplicate your frame as many times as you need.