You could use kit like **broken link removed**and modify analog input to give you slow dimming down function. It is simple to implement and does not require too much experience. From circuit that you posted originally, i would venture out and say that you are absolute beginner in electronics. Because of that I would recommend keeping experimenting with circuitry on line voltage to bare minimum. Untill you master electronics better, I would recommend keeping your interest on voltages you can lick . This kit IS on line voltage, but because it is already proven schematic, you have better chance of finishing it without changing hair style.
If you would like to go that route, modification is very easy to achieve function you need. First you assemble unit as originally intended and test it. After that you modify control voltage input section by replacing couple of components and inserting a switch.