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Open
2 images:

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Copy
and paste the image that you want to use to fill the type [A] onto the
background image [B]: click on [A] to activate it > Select > All;
then go to Edit > Copy; click on image [B] and go to Edit >
Paste
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Check
that the new layer is selected in the Layer Palette and choose Layer
> Add Layer > Hide All. The layer disappears from view.

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Choose
the Type Mask tool and click on the target layer [B], which is the
visible layer. Type the text you want to appear, choose a font and other specifications such as italic, bold etc, then
click OK.
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Reposition
the type selection if necessary by using the Move tool
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Choose
Edit > Fill

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From the Fill menu select: White > 100% > Normal > OK
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To
remove the selection marquee go to Select > None
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When
you have finished, don’t forget to flatten the image and save it as a JPEG
to compress file size: Layer > Flatten Image