[Gimp-user] Cleaning scanned magazine pages

[MrNiceButDim <forums gimpusers com>]

I'm totally inexperienced with GIMP but I chanced upon this recipe that would
meet my needs for a rather big project I'm currently doing. It's about cleaning
scanned magazine pages where the print on the back side shines through the thin
paper making the reading less enjoyable.

Recipe:

With GIMP:

    Open a file.

    Convert your document to grayscale: Image - Mode - Grayscale.

Select the background color: Select - By Color, click with mouse pointer on
the color of the background.

    Invert the selected color: Select - Invert.

    Copy the selection: Edit - Copy.

    Create a new file: File - New.

In the dialog of a new file, in field: Advanced Options choose: Fill with:
White, hit Ok.

    Click anywhere in the window of the new opened document, just to choose it.

    Paste the content of a clipboard: Edit - Paste.

Add a new layer to enhance the black text: Layer - New from Visible, in the
layer's palette, in field: Mode: choose Multiply.

    Combine two layers: Layer - Merge Down.

Save the result as a JPEG file: File - Export As, choose jpeg and set the
quality at least 60.

I have a couple of questions.

A. When getting to "Combine two layers: Layer - Merge Down." the Merge Down
option is grayed out so I can't apply that step.

What could be the reason? And why does it say "Combine two layers" when there is
three?

1.Floated selection (Pasted Layer)

2.Visible

3.Background


B. When selecting a background color to be erased/turned to white what would be
the best way to pick a wider range of grayness than the rather sparse
instruction the recipe suggests "By Color, click with mouse pointer on the color
of the background."

-- 
MrNiceButDim (via www.gimpusers.com/forums)