How to make an antique style postcard.
To start, create a new white shape approx 6x4” on a 3600x3600 transparent background. This layer is is going to be the postcard,
This is how to create the layer (call it “card” and make a large rectangular selection. When doing so, you want to select “fixed aspect ratio” as your rectangular marquee tool’s style and enter a width/height ratio of 4:3. For reference, my rectangle is 6x4”. Fill the selection with white.

Next, Ctrl+Click on the card layer to load its
selection and then go to the Paths palette and
click the “Make work path” icon to create a
rectangular path from your selection.

Now create a new layer call it brush layer and hit “D” to reset the brush color. Now, select the paintbrush tool and open the brushes palette. Choose the 36 diamond brush and set the spacing to 100%. and the size to 100px
Now go back to the Paths palette and click the “Stroke path with brush” icon.

Next, Ctrl+Click on the layer to load its selection. Delete the layer. Now, with the “card” layer active, hit delete to give the edge of your postcard a nice, scalloped edge.

Now make a style to complete the aged postcard look
click fx
choose drop shadow settings as follows
Blendmode multiply
opacity 20%
angle 120' as default
global lighting leave checked
distance 27
spread 12
size 3
then choose inner shadow
choose your color I have gone for a sepia color
opacity 26%
distance 5
choke 13
size 191
Go next to stroke and make settings
size 1 px
inside
blendmode normal
opacity 46%
fill type color
choose dark brown or black for the stroke.

This is how the finished plain postcard looks. Of course you can add your photos to it or lines for journaling and a stamp. Choice how to gussie it up is yours.