Scalloped edges

It looks like they covered the entire cake in white fondant then went back and covered the top and let it drape down the sides with the blue , then just cut the wavy pattern.

I would do exactly as paulo0712 has said above then smooth the waves with my fingertips to round them off. I have done similar for a christmas cake and it looks just like this picture hope that helps

Do you cut the wavy pattern when the blue is on the cake? And thanks for the tips Bevwilliams and Paula0712

Looks like fondant cut in waves.

Yep, Just measure the size you will need with a cake board the size of your cake, Then cut the waves to desired length and look, then lay it on top of the cake.

Thanks so much everyone for your advice