How to apply Prewitt Edge Detection algorithm on a given image?
I was able to answer my question after doing more research. The key is to slide the 3x3 kernel and perform Gx and Gy (gradient for the x and y directions) and calculate the output image values. For example, to calculate the output image top left value -

[x - -]
[- - -]
[- - -]


we have to calculate both gradients (x and y)->

Gx

[-1 0 1]
[-1 0 1]
[-1 0 1]


we take the first chunk of the original image (e.g. sliding the kernel over the top values of the input image)

[1 2 4]
[5 2 5]
[5 4 2]


Gx = 4+5+2 - 1-5-5 = 0

Gy = 5+4+2 - 1-2-4 = 4
We take the absolute values of both and sum them so |4| + |0| = 4

And this is the first top value of the output image.

[4 - -]
[- - -]
[- - -]


The same way we calculate the rest. I hope that will help anyone else facing the same problem.