Ok then, here goes
Make the alpha like I said before, So you should have 2 images open, The helmet design and the new alpha that you just created. I´ve not saved the alpha, so it´s called image1.
Now create the mask from the alpha image.
A box will appear with all the available images open to create the mask from. Image1 is the alpha image so select this (or whatever your alpha image is called).
Your main texture image should now be slightly transparent. This means the mask is applied. Now save the mask to the alpha channel.
When the new box open, you should only have 1 thing listed. If there are more then you already have an alpha channel saved. You can delete old alpha layers with the delete button. For now click OK.
It now ask´s you to name it. Just click OK.
Now you have created a mask and saved it to the alpha layer. But you now need to delete the mask, as it is not needed anymore.
When you click delete, a new box appears. Click NO. Otherwise the mask is merged to your image and you have a semi-transparent image.
And that´s it, done. Just save your texture as a TGA.
I´ve got a good mask saved for doing helmet alphas in PSP7, if you want them PM me your email and i´ll send them across.