adding alpha channel

I am using one of the templates where some colored arrows go across the screen over a white BG.
I want to use this arrows as a transition over my footage.
How do I create an alpha channel so the white BG disappears?
So far I've keyed in FCP but as expected the result is mediocre.

Why don't you just delete the white background? Or, if it's created in Edit > Project Properties, set it to Transparent.

The BG isn't independant meaning that I don't see a layer for it.
I tried to go to Edit/Transparent but it's still not working.
Once in FCP, do I need to ctrl click on the animation and set the composite mode to Alpha?
Thx for helping

FYI you're tip worked out fine, thanks