create a new layer over your base white layer
make sure you select 'transparent'
draw on that layer, and make sure any other layers you use are also transparent.
delete the base white layer.
transparancy!

you MUST save it as a PNG though, and don't flatten the image 'merge layers' but on't 'flatten image'

hope it helps, if it doesn't work, I'll livestream and show you how it's done in photoshop since it's the same basic idea~