To make the button garland, cut a piece of yarn that measures the circumference of the cake x2. String buttons onto the yarn.
Place one end of the yarn at the base of the cake, lightly pushing it into the border. Move around the cake pushing the buttons into the frosting and lightly swirling the yarn as you go. Clip the end off of any extra length and tuck the end behind the last button. Remove the bottom buttons right before you cut the cake.
For the cake banner, roughly measure the distance from one end of the top of the cake to the other where you will place the straws. Using a small piece of tape, adhere the yarn to the top of the straw and wind it around twice. Thread buttons onto the yarn, knotting the yarn between each one (so they won’t slide). Finish by wrapping the end of the yarn to the remaining straw and adhering with a small piece of tape.
Leave enough slack so the buttons will dip down in the middle. Place the bottom end of each straw into opposite sides of the cake.