Separate colors and types if you are like that!
This is how it should look finished!
Use this kind of elastic bought at a craft store.
Line up the buttons and the feed the holes diagonally in an under/ over pattern.
I have dark in the back and light in the front for contrast.
Tie off with a double knot and pull tightly.
This is close up of my Dads WW 2 uniform buttons.
The back ! Hey is reversible...
Add a dot of gorilla or other glue to strengthen the knot...