For this little tutorial we will be using Death Wish, which is THE most popular user episode out there for Blood. However barring some very specific mods (NHIS, IRA and FAITD) this should be compatible with every mod out there.
1.- Download the mod of your choice, in this case Death Wish Version 1.7.9 .
2.- Extract the contemts to a folder. In this particular case the contents of the Death Wish Version 1.7.9 zip are contained in a folder called Death Wish 1.7, so we don't need to create a folder for the mod files (maps, .art files, etc).

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3.- Locate the user episode's custom ini file, in this case dw.ini .

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4.- Rename the ini file to blood.ini .

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5.- Now copy the folder to your NBlood root installation folder (which should be already setup with Blood).

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6.- Run NBlood. Make sure the ini for Blood: One Unit Whole Blood (blood.ini) is selected.
7.- Click on the drop down window and select the folder of your user episode. In this case Death Wish 1.7.

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8.- Click on the Start button. If you followed the steps correctly then when you select a New Game you should see the name of the custom user episode instead of Vanilla Blood's.

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9.- Enjoy.

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Cheers.