You really don't have to double cut to bring ducks in. If you can just single cut by saying tick repeatedly that is a good start. Vary up the pattern. Use the front of your tongue to build up pressure. Release that pressure and quickly shut it off with your entire tongue against the roof of your mouth. Just like saying tick.
tick tick... tick tick tick... tick... tick tick tick
Once you can control that you can move on to the double cut, which is the tick-ah sound. Starts off the same exact way by building up pressure with the front of your tongue against the roof of your mouth behind your teeth. After you say tick, shut off the pressure with the back of your tongue and immediately release it, making the "ah" sound. Start off slow.
Once you get rolling your tongue is like its on a fulcrum alternating between the front and back of your tongue shutting off and releasing pressure. The middle of your tongue should stay off the roof of your mouth. Cupping your tongue a little bit helps.