Creat a new document (Ctrl+N)

Size at 800x600px or 1024x768px that suitable for your monitor.

Pick Gradient Tool

Choose this gradient from the Gradient Editor
If you can't see this gradient, please go to "Load..." ~> Point to gradient preset folder
inside Photoshop folder ~> Load "Special effect"

Hold your left mouse and drag from A to B :
And you'll have this result :
STEP 2:

Download and load this brush into your photoshop : Land_brushChoose any "island" inside this brush set that you like :