Last night I conducted a pair of experiments in ranger rolling.
Firstly, I tried rolling a poncho. First I folded it widthwise. I then turned up the bottom, folded it in thirds lengthwise then rolled it down from the top. The resulting roll was a little smaller than the carrying sack. Ranger rolling either gives me a way to do without the sack or provides a convenient and quick method to roll the poncho small enough to fit in the sack.
The second item I experimented on was a poncho-liner. This took a number of attempts. The method I settled on was to fold the poncho widthwise. Then, rather than folding up the bottom I folded under the left side. Rather than folding into thirds and rolling down from the top I made my folds widthwise. I then rolled the right side to the left. Because a poncho-liner is so thick and you are working on a double thickness getting the right amount of turn-under takes some experimentation. About 25cm seems right.
You end up with a bundle that is close to a cube, but a little wider than it is high. It measures about 13" x 8" x 6", which is almost a Fibonacci/Golden Ratio object! This is a different shape to stuffsack the liner came with: thicker but shorter. Once you have the right turn-under figured out ranger rolling the poncho-liner takes less time than trying to cram it into the stuff sack.