Start By opening hammer. ( i hope i dont have to show anyone how to do this)
Create a new map, make the grid smaller - the smallest it will go.
Now create a brush, do this by selecting the white box on the left hand side of the toolbar.
Create a brush by clicking and holding in the top right hand box, drag a box 232x356.
Now right click the brush and click create object.
Now click on the red arrow at the top of the toolbar (selection tool). Press Z and fly around untill you find the box (fly using wasd-normal game controlls).
select this box and then press the face edit tool on the toolbar.
Now a menu will come up.
Click the displacement tab at the top.
Now click the Create button and a small menu will appear, change the power to 4
Click OK. Your brush will now look like a grid. Now click on the Paint Geometry button. A new menu will appear. Move your cursor over the brush, there will be a green sphere.
Click on the brush to make it raise, or if you want the brush to have a dent in it then right click the brush.
Make sure you experiment with the options like the axis, the distance bar at the bottom and the Distance bar.
Yeah i hope this helps you. Knowing some ppl theyll say its not very well explained.