LAYER MASK

[gaystan]

First, I opened my "base" layer (background image).  Second,  I  OPENed AS A LAYER.  Got my "top" image positioned. Added LAYER MASK.  Then  I clicked on ERASER tool > PAINT BRUSH. Selecting the MASK, painting
the area I want to remove (to show background layer beneath). Using the NORMALmade painting. but the MASKED
LAYER has a green bounding box around it. And I can't ERASE  the part of the "top" layer I want to erase. I tried adding an ALPHA CHANNEL. That didn't work.  The ERASE tool isn't working. I'm not sure what I did wrong.  Or didn't 
do properly.   I have GIMP version 2.10 / Windows 10
garystan
document.oncontextmenu = null; document.body.onpaste = null; document.body.onselectstart = null; document.onselectstart = null; document.ondragstart = null; document.body.oncut = null; document.onmousedown = null; document.body.oncontextmenu = null; document.body.oncopy = null; document.body.ondragstart = null; document.body.onmousedown = null;

[rich2005]

Quote: ...I'm not sure what I did wrong...

Add your layer mask (white) to the layer. No need to add an alpha channel, the layer mask does that.

If you have a green border, you turned Layer -> Mask -> Show Layer Mask ON Turn it OFF

You make the layer mask active by clicking on its icon in the layers dock. Active it has a white border. Then you paint on the canvas  Black to uncover / White to cover/ grey(s) for semi-transparency.

A quick demo animation of that: https://i.imgur.com/1p4fjPr.mp4