How to add a class to an existing model, reduce the images or classes and limit the objects it can detect at a time

Question

I am using SSD mobilenet pre-trained data and I want to add a class for it. I want it to detect QR codes since they were not part of the pre-trained data.

I also want to lessen the model's classes and limit the objects it can detect at a time

I have already my labeled images in .xml

Really need help, Thanks!

Searched a lot on the web and I was suggested fine-tuning and transfer learning. Need suggestions and help from you guys.