Open iTunes.

On Macintosh: Go to iTunes Menu>Preferences>Advanced tab>General "sub-tab"

On Windows: Go to Edit Menu>Preferences>Advanced tab>General "sub-tab"

Uncheck "copy files to iTunes Music folder when adding to library". If you don't want iTunes to make copies of your imported files to the iTunes Music folder.

While still in Edit>Preferences>Advanced choose the Importing "sub-tab"

Set Import Using to "MP3 Encoder".

Change the Setting to the desired quality.

Click OK at the bottom of this Preferences window to save the settings.

Next, go to View Menu>View Options and check "Kind".

Drag the file you want to convert into the open Music Library window of iTunes to add it to the library.

Select the file by clicking on the name. That line should turn blue.

Go to Advanced Menu>Convert Selection to MP3.

You then end up with two files in the library with the same name. If you look at the Kind column you'll see that one is the original file type and the other is an MPEG Audio File (or MP3).

iTunes supports drag and drop importing and exporting, so you'll be able to select the file and drag it to your desktop, or wherever you'd like to save the final MP3 file.

You can then delete the files from the iTunes library if they're no longer needed.