CGAffineTransform
work in Quartz coordinate system, that's the point from where you should start:
CGAffineTransform translate = CGAffineTransformMakeTranslation((x,y);
CGAffineTransform scale = CGAffineTransformMakeScale(q,z);
CGAffineTransform finalTransform = CGAffineTransformConcat(scale, CGAffineTransformConcat(translate, assetTrackForVideo1.preferredTransform));
[layerInstructionForVideo setTransform:finalTransform atTime:kCMTimeZero];
[layerInstructions addObject:layerInstructionForVideo];
that kind of transform work for me.
EDIT: my mistake, forget to edit all code !