Photoshop Tutorial : Change background colour and keep shadows

I would like someone to create a tutorial (Video/Screenshot/PDF?) that I can use - so I can edit all future images myself.
Someone to basically walk me through all the steps required - Give me a tutorial that I can keep and use whenever I need it.

I have attached 2 images (one red dress) (one light jumpsuit) so you can see they type of images I will be working with.

I have images that I need to change:

1. Change the background colour from Dark Grey to light Grey (f6f4f5) and keep the original shadows showing.
2. Crop all images so that the models seem to be the same size (as per Dark dress)
3. Reduce the image size to be used on website.

I am an absolute beginner using Photoshop.

I'm not sure what to charge for this - so please do let me know if £20 is in the correct price range for this request.

I look forward to hearing from you all.

Thanks a million!