The KDialogBase dialog class will simplify and speed up your work when you design dialogs. The class supports common dialog layouts such as a tabbed, icon list and tree list shapes as well a blank (or empty) shape that you can use for whatever you need.
Try to make a dialog modeless if you think this will make the work simpler for the user. It can be a bit harder to implement a modeless dialog but you should always care about your user first. Some dialogs, such as a file selector can be modal.
The KDE user interface library (kdeui) contains a number of ready to use dialogs and widgets. Use them! Your users expect to see familiar dialogs for standard tasks such as selecting a font. Use the freely available source code. Look at other programs and don't be shy about copying the code fragments that fit you needs, as long as you do what the license of that programs tells you to do and you credit their authors. But, to make good dialogs, you must nevertheless try to understand why and how. Non optimal solutions, errors, and potential bugs are located more easily this way.