For crying out loud, my friend, you need a tutorial in the use of photoshop or GIMP before you can use any other tutorial.
Anyway
,
Which one do you use?
Did you ever bother exploring them?
Or google the question "How do I resize an image in photoshop/GIMP?"
In photoshop, under the Image tab, there is the Image Size button, which allows you to manipulate the dimensions of the image.
Be warned the resulting image will be of lesser quality.
In GIMP, you can find the same function under the Image tab, in the button Scale Image.
Model manipulation is an art, not a science.
It may have it's technical aspects and it does require imagination, but it still is not a science.
Do you think a painter ever learned his art without getting paint under his fingernails?
You need to get dirty if you want to do this sport.