I collect seeds from my Ornamental Rhubarbs, so I assume the procedure would be of the same for the regular kind.

Just let them flower, don't cut off the spent flowers, seeds will form by turning brown, they must be collected after it is mature, but before they shed. Thoroughly dry the seed for proper storage until your ready to sow in the fall or early spring.

I've had great success wintersowing ornamental rhubarb.

Sharon