First up, take some white felt and cut out two identical clouds.
Cut a piece of hessian (or any other fabric!) and place it between the two felt clouds.
Using a full strand of thread, sew a running stitch around the edge of the cloud through all 3 pieces of fabric, leaving a gap of approximately 1 inch.
Use cotton balls or (as I didn't have any!) torn up tissue to stuff the cloud, then sew up the remaining gap.
Cut the hessian around the cloud leaving a 1cm border, pull strands out or rough the edges of fabric as desired.