Try having a read through the late Dr Sophia Lin's teachings...
https://drsophiayin.com/blog/entry/w...make-it-clear/
This is training how to walk loose leash, which will help your dog to focus on you rather than stop and go.
This one has some good instructions for leash training too...
Leash training your dog - Dogtime
BUT. If your dog has started to mark...it's not going to go away. You'll have to accept it sooner or later - it's a dog's nature thing.