I'm finding it hard to work out how to hire you on an hourly basis without setting a fixed number of hours or fixed cost. I think that this is the way to do it but I still had to enter a number for "hours per week" which i don't know. I think that you use the Freelancer desktop app and it calculates the hours that you spend on this.
I'd be happy if you give this first one or two a go and just tell me how many hours you spent but I'm not sure it is that simple. Let's try this and see how it goes.
Hopefully the next step allows me to send you the details...