1.  Open a picture.

#1

2.  Go to Enhance>Adjust Color>Remove Color.  This makes it black and white.

#2

3.  Now, you need to either adjust the brightness and contrast or invert the colors.  On brushes, anything white will be transparent.  So, to make what is white on your picture black, you need to invert the colors.  On my picture, since the flower is mostly a very light grey, it is necessary to invert the colors.  Go to Image>Adjustments>Invert.

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4.  Now, on my picture, to just select the flower, I use my magic wand tool.  Of course, in so doing, I've selected some items outside of my flower and not selected some items on the inside of my flower.

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5.  To include the stuff inside the flower, I click on my polygonal lasso tool.   Holding down my Shift Key at the same time, I make a circle around the center of my flower.  Holding down the Shift Key adds stuff to your selection.

#5

6.  Now to remove the extranneous stuff on the outside of the flower, I use my polygonal lasso tool.  But this time, holding down the Alt Key at the same time, I make a circle around the extranneous stuff.  Holding down the Alt Key deletes from your selection.

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7.  Now, I just have my flower selected.  If I want to, I can adjust the brightness and contrast of it to get more of the flower actually showing.

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8.  Now, it is time to actually make my brush.  Go to Edit>Define Brush.  Name your brush and click okay.

#8

You are done.  Now, click on your brushes, find your brush and use it.  Here is my brush used on white background.

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