I've translated a few chapters as a hobby in the past and have found out the same thing, tehKitten.
I've found that what works best for me is to first make a draft as close to the original as possible.
Then change it all so that it sounds natural in my language.
Then change it all again to keep as much of the 'feel' of the sentence, eg humour, tone or whatever, even if it is unrecognizable in other respects
A good translation is hard work, and an art in itself. You have to have a great understanding of the language you're translating, to be able to recognise subtle nuances in tone and meaning. And you have to be able to express yourself perfectly in yours. It being your native language is not enough. Oh, and you have to understand exactly what the writer had in mind, to understand as much as possible what they are describing and what they are trying to convey.
I hope I didn't make you anxious. I do wish you success in your endeavour, and I believe it bodes well that you did realise early on how much more there is in translating than meets the eye.
All that said, Maisy May is written in the very natural, informal voice of a 15 year old girl. So maybe you are in exactly the right position to make a very beautiful and natural-sounding translation.