First go into the editor. Click the Map <untitled> area and click new to create a new map.
Then press F3.
Go to the "entities" section and find vehicles.
Find any helicopter you wish (some don't have weapons).
Then, while still in entities, go to human and find heli pilot.
Place 2 next to the helicopter.
In the bottom right press start!

Now after pressing start select the pilots and put them into your new helicopter.
If they don't both fit that means the helicopter can only hold one person which is fine!
(Some helicopters require 2 pilots, one for movement one for weapons)
After they have boarded their helicopter select the helicopter and hold CTRL for direct control mode.
While holding CTRL press space. This should start the helicopter.
While still in direct control mode hold PAGE UP to increase the altitude of the helicopter to what you want. Once it is at the desired altitude you may now move your helicopter like a normal vehicle!
CONGRATULATIONS!

If your helicopter has weapons you may use them.
Just a few things to remember when using your new toy:

• Your Helicopter is weak and enemies can shoot them down real easily.
  
• Your weapons are angled so you might have to position your heli in what seems to be a weird position.
  
• Reloading rockets takes a long time.

Now that you know a few things here is an example of a helicopter fighitng a small band of VC AK47s.

Sadly here was his fate...

Some tips, if you select who you want to shoot at and then move your helicopter it will do a strafing attack!
*Doesn't always work as sometimes the helicopter moves too to target enemies on the ground.

You can transport units in the helicopter but you can also load/unload units while the helicopter is flying.
*Note the Co Pilot (if any) will leave when you click unload.