Let's do a quick tutorial.
Let's say you want this slider, it goes from
(Image 1)
over to
(Image 2)
and finally to
(Image 3)
Then you first of all need a slider object. As X/Y coordinates for it, you just set the place where you want it to be. As slider image you set a blank image, as the whole slider rendering will in this example be done by the background.
The slider points would be: (0,15),(200,15)
As thickness you could chose 15.
(Image 4)
(0|0) is here the (x|y)-coordinate of the slider.
Now add a slider background and open its slider background generator.
Select a horizontal slider.
The width is 200, height 30. In this examples the filling of the slider has no margin to the background, so set all margins to 0.
Next split up the slider filling image so that you have the following three images:
(Image 5)
Now go to the next step in the slider background generator.
As background image choose the unfilled slider as shown in image 1.
As left edge image choose the left edge of the slider filling, as depicted by part 1 of image 5.
As middle image choose the center part of the slider filling, as depicted by part 2 of image 5.
As right edge image choose the right edge of the slider filling, as depicted by part 3 of image 5.
Then hit generate and you're finished.
If your slider has no specific edges you might not need to split up your slider filling and just use the filling as "middle image" in the slider background generator.