You will need to finish the edges of your postcard.  I like to cut strips of fabric to cover these sharp edges from the can.  You could also use bias tape, quilt edging or something like that.  When I use fabric strips I just cut strips about 2 inches wide.  I really don't measure because I trim them and leave the final side raw on one side because I like the way that looks.  On my machine I have to have fabric facing down for my card to feed through the machine so it is easiest to start with the back of the card facing down.

The aluminum can is facing up with the fused fabric back side of the card facing down.  I lay the right side of a fabric strip along the long side of my card and stitch.  If you run the side of the card along your presser foot this makes the trim a nice width.  Once I have stitched a strip of fabric to both of the long sides of the card I press the trim flat, away from the front of the card so that the right side of the fabric is facing up.  Next I fold the fabric over the edge and overlap it to the back of my card and press it down nice and flat.  This makes it much easier to sew.

Flip the card over and you will see what a pretty edge you have on front!  Next you must sew along the fabric edge, right next to the can.  If you 'stitch in the ditch', right along the fold between the can and the fabric that is fine also.  It's up to you!  Finally, turn the card over and trim the raw edge away right next to your stitching.

I just snip the ends off of my trim before I start the short sides of the card.  I repeat the same process for the short sides that I did for the long sides of the postcard.  Start with the right side of the fabric facing down and proceed.  Don't worry about it if you get it wrong.  One card I made had one of the three sides with the wrong side of the fabric facing up.  So what?  These are unique cards and maybe you planned such random changes.  Only the artist knows for sure!  I snip the ends off most of the time but you can finish the corners if you are patient and skilled enough to do that.  I am not!

Here are some of my finished cards!  I like to stamp the backs and mail them in plastic sleeves.