Hmm, perhaps the instructions aren't in there... I will check into that. Basically, the way you would make new fields is by duplicating the "extrafield" throughout the files.
For example, in this file:
includes/templates/YOUR_TEMPLATE/tpl_account_edit_default.php
You would find this code and duplicate it, then add your new variable instead of "extrafield". Let's say, "extrafield2".
<!-- start extrafield -->
<label class="inputLabel" for="extrafield"><?php echo ENTRY_EXTRAFIELD; ?></label>
<?php echo zen_draw_input_field('extrafield', $account->fields['customers_extrafield'], 'id="extrafield"') . (zen_not_null(ENTRY_EXTRAFIELD_TEXT) ? '<span class="alert">' . ENTRY_EXTRAFIELD_TEXT . '</span>': ''); ?>
<br class="clearBoth" />
<!-- end extrafield -->
Do the same for all the remaining files... find the variable "extrafield" and copy then modify the code.
Then, you'll want to run the Install script for your new field. Swap out "extrafield" for your new variable in the install.txt script, then run in SQL patches.