Using the jello packets (just the powder, do not make jello) and/or cocoa powder, place some glue on the index card and sprinkle the powder onto the glue. Let it dry. Then place it in it's own plastic bag. You will need to take it out of the bag for the child to smell the scent and see if they know what it is. You can also draw a picture of what it is on the back (or print out a picture).
Craft sticks/popscicle sticks work best for wet scents because the wood holds onto the smell. Dip about 2 inches of the craft stick into the desired flavoring (vanilla, almond, orange, mint, etc) and let it soak for about a minute. Then, let it dry. Place it in it's own bag as well (you don't want the smells to start to mingle). You can write the smell on the end of the craft stick.
Ta Da! You have yourself some scents for your sensory box!