Prepare bungees as shown in the picture with a loop on one side and a knot on the other side.

The loop should just be big enough to fit over your riser and not fall off.

Put the loop around the bottom of the riser as shown in the 2nd picture.

Do not pull the open end of the bungee through the loop at the bottom of the riser - the open end should come out to the 'outside' of the riser.