Press Spacebar to jump
Use the arrow keys or w,a,s,d for movement.

Click E to enter edit mode!  Select an area of land with your mouse and then use your mouse wheel to morph what you made!  Click E again to return to world mode and walk around in your creation.

Hit ESC to enter the menu - There are a lot of options in the menu such as adding cool new objects such as jump pads!

Press H to enter heightmap mode.   You need to have an area selected then scroll the mousewheel to make hills and mountains.

To Make Ramps/angles:
Use your third mouse button or press the mouse wheel and drag it across the top of the block you want to make the ramp from.  You should see white boxes being formed as you drag.  Then scroll the mouse wheel forward until you have your desired angle.

To Make Water:
Select where you want the water and then hit ` to enter the console.  Type editmat water then enter.  Then scroll forward on the mousewheel.  To make deeper patches of water G + mousewheel to increase the grid size and then use the editmat command.

Models:
Enter edit mode and select where you want the model to be placed.  Then hit ESC to enter the menu then select editing/ents/mapmodels  Models will appear dark but if you hit L to turn on fullbright you can see them. A simple way to duplicate the models fast is to select a created model and hit C  Then select where you want the new one and hit V

To Save Your Map:
hit the ` key (above tab) then type /savemap mapname 
mapname is the name of your map
Your Map will be saved in the Base folder.  Save often!!

Loading Your Saved Map:
hit the ` key (above tab) or T then type /savemap mapname
mapname of course is whatever you named your map when you saved it

Screenshot:
Press F12 ! You might want to convert the image to a different filetype using a free program like ifrit or The Gimp since screenshots are saved in bmp which are big in size.