How to project coordinates annotations from the image space to the feature space?

Let’s say I have an RGB image (3003003) with detections annotations. I am performing feed forward of this image into an SSD 300300 (with a VGG backbone). I want to translate the annotations coordinates from the image space to the feature space of the VGG backbone at the 51238*38 feature map layer.

How do I project the annotations coordinates from the image space to the 5123838 feature space?