These are some simple black icons which you can print onto adhesive paper or decal paper and then add to your Interface keys for clarity.

I used plain adhesive backed paper in an inkjet printer which once printed I covered with a clear plastic for durability. These were just the materials I had lying around the house so there are better ways i'm sure. Including the option of decal paper which would be more permanent.

Cut around the dotted line and stick onto your keys.

Done.