OK,
The easiest way is to go to Freeflight and then select an airfield that you have installed and is in the ETO. Second would be to install a mission made up for the ETO and select it from the single mission menu.
Let me assume that you have not installed any ETO infrastructure and let's get started. The ETO must be installed as optional scenery and airplanes. It's all fairly easy and straight forward and is a gradual process.
I guess the best way to explain it is to list the add ons that you'll need. Almost all of them are in the library here. All come with instructions to install them. Once you get the airfields installed, the CFS 2 program will give you the option of flying from those airfields in the Freeflight menu.
What you'll need:
1. Airfieds. 2TAF or Achim's fine aerodromes to start with
2. Mesh (mountains and valleys).
3. Textures for Europe (SDC or the new FS X conversions)
4. Airplanes (German-French-British-USAAC-Romanian-Etc.)
We can use this thread as a "how to" in order to get started and flying in the ETO.
First: Go to this link and download the British aerodromes by Achim:
http://www.sim-outhouse.com/index.php?lloc=downloads&loc=downloads&page=info&FileID=12910
Install per the instructions and activate through the Scenery Library in CFS 2. These airfields will require you to install some other scenery files, but the readme file by Achim is very well written and will give you all the links you need to do so. Take your time as once these extra files are installed, they will also work for airfields all over Europe and the MTO.
Second: Install these two aeroplanes:
Do 17z-2:
http://www.sim-outhouse.com/index.php?lloc=downloads&loc=downloads&page=info&FileID=9409
Hurricane Mk 1a:
http://www.sim-outhouse.com/index.php?lloc=downloads&loc=downloads&page=info&FileID=2996#download
Once you have these installed, post on this thread and I'll make up a quick mission for you to play with and will get you over London circa the late summer of 1940.