I saw a recipe on Jello.com for these and it had you putting lime jello mix into the cupcakes and frosting etc. I thought that sounded kind of icky so I did my own thing. Here is what I did:
1 box white cake mix (along with ingredients to make the cake mix)
1 can vanilla frosting
green food coloring
1. Make cake mix according to package directions. Add a few drops of green food coloring and mix until you get the desired shade of green.
2. Bake according to package directions for cupcakes.
3. Put frosting in a bowl, reserving about a tablespoon aside to leave white (you will use this as glue for the blue m&ms later on). Add a few drops of green food coloring to vanilla icing and mix well until you get the desired shade of green.
4. Frost cooled cupcakes with green icing.
5. Slice each large marshmallow into thirds so you wind up with three discs (so one marshmallow will do 3 cupcakes).
6. Slice mini marshmallows into 8 pieces (you need one whole one for every cupcake).
See pictures below for how to assemble.