We need the coordinates of the airfield and the reference altitude.  Some information about the runways would also be a help but there is none of that here.   Fortunately the airfield is quite clear on Google Maps so we can get started from there

From here I can determine:

1. The location and general layout of the airfield
2. The coordinates of the center of the airfield which I will use as the reference point
3. The altitude at the reference point

and that is enough to get me started

1. Use the Google Earth ruler to establish
2. Length which is 354.1 meters
3. with a heading of 145.55 degrees

I used the ruler across the runway to get the width.  It varied a bit which is not easy to handle in the simulator so I set it to 20 meters

Go to the main menu  File > New Airport

1. Be careful to enter the coordinates correctly.  Generally if your new airport is not there then it can be attributed to wrongly entered coordinates.  ADE will accept coordinates in most common formats.  From Google Earth I just moved the N and W from the end to the start
2. Enter the altitude remembering the units you have set for altitude.  Beware of entering meters for feet and vice versa
3. This airport does not have an ID so we need to give it one that is not in the Simulator.  If you do use a stock one then ADE will not let you proceed
4. Enter the country.  You might want to check how the country is displayed in the simulator so that your new airport is assigned to the correct place.
5. Talgarth is not really a city!
6. Finally give the airport a name
7. This is being created for FSX or P3D so the traffic scalar and test radius are shown.  Just leave these at the defaults and click Save

Use File > Airport Properties to re-open the Property Dialog

1. Note that ADE will find the magnetic variation and Region ID for you
2. Note that the Stock Airport check box is not checked.  ADE knows that this is not a stock airport
3. The coordinates are now shown in whichever display format you selected for ADE.  In this case they are decimal.
4. There is not much to see at the moment but ADE has created the airport reference point and put a tower viewpoint there.
5. Click OK or Cancel to dismiss the dialog

Make sure you save the project now.

ADE will give you a name but you can change that if you wish.  Click the Save button to save the project.

Next jobs will be to:

1. Add the runway.
2. Add a background image to help lay things out
3. flatten and clear the airfield area

1. The airport background polygon
2. An apron to represent the grass area
3. The runway

Needs more work but here we are.......