What you need:
push pins or packing tape
Clip the clothes pins to the twine by sticking it as far into the clothes pin as possible. This will keep the clothes pins from sliding around. Position the twine on the wall where you want it (it helps to have someone, like Auntie K, hold the other side for you!). Tape or push pin the left clothes pin to the wall. Leave room for swag and repeat on the opposite side. Easy peasy.