I folowed the steps given by Fabrice Henrion in this question (https://www.odoo.com/forum/Help-1/question/Is-there-any-way-to-preview-WebKit-report-layout-while-coding-maybe-WYSIWYG-Editor-52307#answer-52417) and i am now able to edit the texts in the HTML report.
However, i would like to know how to edit the layout and add functions fields, for instance the amount_to_text function to the invoice/order.
So if somebody knows how to do that, it would be great.