Does the BBox labelling for object detection has to be done manually on Images or is there any ways to automate it
Short answer: You will probably have to do a lot of it manually, or maybe hire an intern. 
If you already have a way to automatically annotate your data, you don't have to build another. 
You can try to use another way as an initial Solution, which you still need to go through by hand afterwards and finetune or correct the solutions. Depending on the size of the project it might be worth it though. 
Ways to do this could be things like using annotated data for similar classes, using a non-deep-learning way that kind of works, using video footage where you know consecutive frames having similar objects and bounding boxes so annotating is faster.
I'm doing a project on custom object detection. I have to use a Bbox labelling tool and yolov3 weights to train my dataset. Still I'm confused about where to start and proceed with the same.