I can tell how I did it on my transmitter. Different brands and even different transmitters within a brand may have a different way of doing it. I might be able to help on your specific transmitter if you tell me exactly what you have. Sadly, many of the lower end transmitters do not have this function.

This is my 10th season using my rock solid Futaba 12FG radio. In my case, I go into the "Linkage" section of the menu and I select "End Point". It is on the End Point screen all the way to the right hand side to the column labeled "Speed" where you can slow the servo down. See the attached photo below. You can see that Elevator and Aileron have a value of 6. For flap settings, the number is best anywhere from 18 to 20 which makes them quite steady and slow. You certainly don't want that much on flight surfaces. I started with a value of 3 which helped but went up to 6 which is even better. "0" is the default value where the servo is not slowed at all. I can slow down any or all 12 servo channels if needed.