You will need about 20 minutes for the tutorial. Then you will be familiar with Designer elements.  

Designer is working in conjunction with GuiXT. So, when you want to modify a screen with Designer you have to start GuiXT before logging on SAP R/3.
Being in the screen you want to modify, you can call Designer within the tool menue from GuiXT
or by the designer icon:

 

You will open a new screen you can work on with Designer:

  For each screen you modify Designer will write a script and store it in the directory specified in the guixt.ini file.
Details can be found under Documentation.