start slowly from one corner and slide your thumb over the sticker so you have as small/thin folds on the surface as possible, don't apply any pressure to this area at the moment...
then take a sharp penknife or stanely knife, and very very very very very gently cut and fine sliver out of the sticker along the fold of the sticker, so that the sticker is flush with the curve of the helmet, harder to explain than to do mind...
then apply pressure to the sticker, so the folds are now flat and flush, pop any bubble with a pin/needle/razor blade
works best if you can manage to get the crinkles/folds over areas of block colour, and not writing/patterns or else you will have to try and match letters up and shit.... try and arrange your crinkles correctly!
but obviously be careful with sharp shit around your lids because if you score a line in the hard shell your helmet could crack along that line in a bad crash, not goooooooooood!.