First set up the min and max properties, like 0 and 255, either in GUIDE or in your code. Then, when you need to use its value, get the slider value from any callback like this:
sliderValue = handles.slider1.Value;
Then use it to binarize an image like
binaryImage = grayImage > sliderValue;
imshow(binaryImage);
or however you want to use it.
