go and buy a flat file, with a serreted edge, from the pound shop
and take 2mm of each side
how you do it is thus
-file one side down to the required amount
then bolt it in the rear forks, so the wheel is sitting outside the frame - wheel nut inside the frame
-then you can get the same positioning as the side you first filed, as you have the stay to go by
-you will just need to buy a file from the pound shop, that is the same or slightly greater in thickness than your forks
however in the front, you will need to open the landing gears to 14mm, if you aint got 14mm ones
that will entail both a 14mm bit, and filing still, as you want a slot, for safety