Last day, I happened to surf the web on this topic - Onnara/ Thaar. Surprisingly, I got a description (may not be excellent ... and something is better than nothing ! ) in one of the sites. I copied the text and and it's here for you.
How to wear an Onnara.
Onnara is one and half sized mundu, nearly six muzham in length. This is wrapped over the buttocks and the two free ends are kept at equal length. One end is pulled up to the face level and the other end is tightly wound over this and the end inserted at the right side of belly. Then leave the top end and it will drop the other end and the conical end will be touching the ground almost. Now kneel down and catch the back portion of the mundu and bring it up to the waist under the conical flap portion dropping down. Now spread the legs little bit and insert this portion in between bottom belly and the edge of the dhothi. This portion will be tight now over the pelvis, the thighs will be covered upto the knee fully and partially downwards. Now the final process starts. Bring one hand to the back and catch the dropping conical front end through the spreaded legs (between thighs) and pull the end as much tight as possible and nicely insert the tail portion in between the back over the buttock and cloth.
By now for ladies of some good structure some statue look will be there and curves and shape of the back will be well displayed. In olden days only a plain dothi used to be worn over this so that the onnara will be partially visible through the cloth. Now there is one underskirt for fully covering the onnara. Still the buttock shape will be prominent with an onnara and any brand of panty can't keep the structure like this.