A basic process to navigate Gimp for Windows.
File >> Open find your File from the hard drive/folder to manipulate.
1) Place your mouse/pointer on the picture:
Right click: Image>> Colours>>Auto>>Normalize or Stretch Contrast.
This corrects your photo automatically.
Level the horizon
2) Right Click: Tools >> Transform Tools >> Transform.
Click and rotate the picture to get your horizon straight.
Drag the picture then press "rotate"
3) Right Click: Tools>> Transform Tools>>Crop and Resize
Click and drag the scissors to crop as desired.
4) Right Click: Image >> Colours >> Colour Balance.
Chose from Saturation, Colour Balance, Curves, Contrast etc to suit.
5) Right Click: View >> Zoom In/Out lets you see the picture enlarged or smaller
6) Right Click: Filters >> Enhance >> Sharpen or Unsharp Mask.
The amount of unsharp masking depends on the scan.
Radius 1.0 Amount 0.80 Threshold 12 is a good starting point.
Sharpen >> default is 10. Dont increase more than 40 or you get jaggies!
7) Right Click: Dialogs>>Brushes. Chose a small "fuzzy" brush. Refesh.
Press letter "C" on Keyboard. Place the pointer over the sample colour you want with a left click and press "Ctrl" on the keyboard at the same time.
Then "clone out" dust etc with a left click over the specks of dust etc.
8) Right Click: Image>>Scale Image. You will see the width in pixels.
Change the width value to 1024 or whatever you prefer. Press "px" and see the new size after pressing OK.
Saving as a JPEG
9) Right Click: File >> Save as >> By Extension >> Chose JPEG.
Check/change the file name in the bottom of the dialogue box if necessary, then OK.
If the programme asks you to "Export", press "Yes"
Move the Parameter Setting "Quality" to 1.00 (Right hand side) to get the best jpeg and a high file size. If you move it left, you can see the picture deteriorate.
Don't touch "smothing", keep it at 0.00
This is a quick rough guide, but like all types of software, play around with it.
There's lots of other nice effects installed, have a look.
Also see the on-line user manual for the other effects.