As my work is very diverse and therefore time and effort diversify as well I agree upon the fee individually with every client. As a basic principle charging happens mainly via an hourly rate or a flat fee. The current hourly rate is 52 Euros without VAT as I am (still) a small business.
For an exact overview of my working time I use the time tracking tool Clockodo
which I will provide with every invoice. In this way I can guarantee a precise accounting.