Yaye Managed to fix it all by myself:)
My phone is: C510 Black operator 3 ans Swedish

So this is how I did and what files I used:
Main C510_R1FA035_MAIN_GENERIC_KT_RED53.mbn
FS C510_R1FA035_FS_NORDIC_RED53.fbn
Custpack C510_1220-9636_R9A_3_Sweden

Flashing Main and FS was not really any problem, once I got some power in the battery
It was the custom files that gave me some headache. I got the turtorial on 1:st post in the thread all the way to clicking the FileSystemTool button, but then I got confused. Until I kinda started to look outside the box. This copy file from... to...??
To the right pannel double click on "TPA", then double click on "preset", then double click on "custom". Select all the files from the unpacked CDA and "drag and drop" them to "custom" folder.
So I did as said above but then what??
I then realized that I had to go to the place where I unpacked the CustomFiles on my computer (in my case on my C disk, under program and in my Sony Ericson folder) and then double click on the folder until got to folder named "Custom" and copy all files who was in that folder and paste them into the A2 program in the right panel as said above.

Now my daughter finally have her mobile back from surgery, with transplanted brain. Great:)
Want to thank all who posted in this thread, cuz I have read them all in my hunt for answers and even got back on sum to check certain things. This is so nice and my daughter got so happy when I told her that her long lost mobile finally showed some kind of life form again.