It took me a while to work out how to create the Alpha but, like most things in life, once you work it out its actually easy.
In an effort to assist those new to the skinning game I have written a tutorial that may help:
To create my skins I use Paint Shop Pro (currently 2019 version) purely because thats the program I have become used to over the 15 years of skinning for the old IL2 and DCS World (on and off).
However with these skins for IL2 I had to learn to use GIMP as Paint Shop Pro is not compatible with the Intel DDS converter.
So I use Paint Shop Pro to create the painted, layered colour skin and GIMP to create and manipulate the Alpha layered template and save as the finished DDS skin.
My process for creating the individual Alpha template for each skin is as follows. (This assumes you have a seperate Alpha template already like the ones that come with the ICDP template for the P-51D/P-47, my particular area of interest).
NB: I am not saying this THE way to do it, there may well be a better method that I am unaware of, but this is merely the method that I have found that works.
You may know most of this already but you may also see something that you are missing in your creative process:
1/ Open both the colour and the Alpha templates together in GIMP so you can switch between the two.
2/ Create an empty layer on the Alpha template that corresponds with the layer you want to convert on the colour template.
3/ Copy the colour layer from the colour template and on the Alpha templates corresponding empty layer select "Edit/Paste in place". Your colour layer will transfer directly into the layer in exactly the correct location.
4/ You now Right click on the image, in the menu that appears select "Layers" and in the next menu left click "Anchor Layer".
(You should now be looking at a grayscale template with a colour layer in place. We now need to reduce that colour down to its grayscale levels and adjust their brightness to achieve the desired relectivity) :
5/ On the Alpha template, making sure your new layer is the active layer, go to "Colours" menu and select "Saturation" and drag the slider to reduce it to zero. The colour will now change to grays.
6/ Still in the "Colours" menu, select "Brightness and contrast" and reduce those sliders to zero. The images on your layer will now go to solid light gray with no detail visible, just the shapes.
7/ Still in the "Colours" menu, select "Curves". This gives you a small screen with a graph that has a diagonal line running from the lower left (the white value) to the upper right (the black value) corners. By clicking and dragging this line you can manipulate the amount of Black an White you input into the image on the layer. The effect on the image is immediately visible. (Also available just above "Curves" is "Levels" which does the same thing but uses a slider. I just find the "Curves" method more intuitive)
8/ Repeat steps 2 to 7 for each layer you want to manipulate.
(As an example in the image below of "Down For Double", The metal finish, exhaust stain, the white paint, red stripe, noseart, rudder paint, codes, serial numbers and insignia are all on seperate layers on the Alpha and adjusted individually in this way.)
9/ I then "Export" the finished Alpha template as a PSD file to the relevant work folder I use to store them, but if you just use Gimp then you will "save" them in the Gimp format. Either way the Alpha template will still remain open in Gimp.
(So you should now have built a grayscaled copy of your main colour template with each layer manipulated to achieve the tones of gray to achieve the desired level of reflection. Now we need to combine both templates and export them into the sim as a .DDS image):
10/ With the Alpha template selected go to "Image" and in the drop down menu Left click on "Flatten Image". All the layers will now merge into a single layer with the name of the last layer at the bottom of the layer list. (in my case it is called "Base", you might have called yours something else, but for this tutorial I will refer to it as "Base").
11/ The layer menu should now consist of a single layer, make sure this layer is active and go to "Edit" then "Copy".
12/ Now switch to your main colour template. make sure one its layers is active (it doesn't matter which one) and go to "Image"/ "Flatten Image". (Your Main Colour Template will now also merge into a single layer)
13/ You should now have a single layered Main Colour Template. In the layers list aove the table of layers you will see three tabs: "Layers"/"Channels"/"Paths", Left click on
14/ You will now see the current Alpha channel for your Main Colour Template showing its Red, Green and Blue layers highlighted with the overall combined "Alpha 1" layer at the bottom.
15/ Left click on "Alpha 1". You should see the Red, Green and Blue channels "switch off" and the "Alpha 1" layer should now be active.
16/ Now press "Ctrl and V". This will now paste YOUR Alpha layer that you flattened and copied at steps 10 & 11 onto your Main Colour template and will show in the channels list immediately below "blue" channel. (so the list should now read "red" "green" "blue" "Alpha" with the layer "Alpha 1" at the bottom).
17/ Left click the "Layers" tab adjacent to "Channels" and in the layers table you should see a new layer above your "Base" layer that says "Floating selection". This means your new "Alpha" channel is on the image but needs merging.
18/ To merge them Right click on the main image and select "Layer" then in the drop down menu select and Left click "Anchor" layer. You should now see that the "Floating Selection" layer has gone with the single "base" layer showing.
19/ Make this single "base" layer active.
20/ Now go to the "Layer" option in the toolbar at the top, Left click and in the drop down menu select "Mask" then "Add Layer Mask" (if this is grayed out then check you have clicked on the "Base" layer to make it active).
21/ The "Add Layer Mask" menu will appear in the center of the screen with the "Channel" option already selected (if for some reason it isn't then select it) and "Alpha 1" will be in the text box immediately below.
22/ Now Left Click on the "Add" button. The image will now go transparent with a check pattern all over it. (This is normal, do not panic!)
23/ Now got to "Files" then "Export As..." (***DO NOT SAVE!!*** or it will combine both templates permanently and you will have lost them!)
24/ The "Export Image" menu will open. Give your skin its name including .dds at the end in the box at the top, then use it to navigate to the drive you have il2 installed on (the path is usually: 1cgames folder/il2 sturmovik Great Battles/data/graphics/skins/planes/(select the relevant plane folder).
25/ Now the "Save as .DDS" menu will appear. Select the following:
"Compression" and then select "BC3/DXT5" in the menu.
"Mipmaps" select "Generate Mipmaps"
Select "Advanced Options" then select
"Filter" and then in the menu select "Box"
Then click "OK"
The above are the only options that need to be changed here.
26/ The completed .DDS image will then be exported to the sim skin file selected.
27/ When its finished, (it just takes a few seconds), the "Export Image" box will close.
28/ Close down GIMP. The "Close Gimp" box will appear, select "Discard Changes" and the program will seperate both templates again as it closes.
29/ Either start the IL2 Viewer or start the sim itself to see the results. I use the IL2 viewer to check alignments etc and use a Quick Mission in the sim to see the actual finished aircraft. (In the case of natural metal finishes I always use the Quick Mission to see the result as it always looks different in the IL2 Viewer and the sim "Hangar".
As I say that's my method, and someone may have a better way, but it works.