you don't need to tie on a rock, just make sure the wind is at your back, tie on a fly coil 30-40 feet of line and leader in your non-rod hand throw it into the air and then when the wind takes it straight out, lay the rod tip down to the water. The alternative is a modified roll cast where instead of rolling it you just lift the line off the water high enough for the wind to catch it then you quickly raise the rod tip causing a wave to form in your line which the wind will Carry out the length of your line. give it a try I am sure you will enjoy the results.

Eric