edit jerseys with alpha channel

Hi..someone who can help me to understand how to use the alpha channel? ...I use GIMP

how to apply the alpha channel I understood...but obviously there is something wrong because I can not get the right result
I explain:
I extract the bitmap texture from JERSEY EDITOR 1.1..and I open this with gimp.
this is the bmp texture that I edit


and this is the result that I should be getting ( this is a default texture with alpha in my GIMP...from IFF EDITOR beta)


this is the texture alpha present in the default texture ( I extract from it)


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now....I can not turn my texture BMP in the right texture PNG with the right alpha channel.
there someone who helps me understand, step by step, how to do??.......

thanks to all

Hello,
I've had the same issue.
I must use Photoshop to edit the jerseys.

right click the layer

add layer mask

choose an option (youre just going to import one anyway)

right click the layer again

show layer mask (you dont need to do this, but it helps so you can see it)

paste in your alpha

anchor it

right click the layer

apply layer mask

save, smile, be happy

thanks jaosming..

no problem, not many people use GIMP here, gotta help my borthers out