From what I found in generalist books about the XVIIIth Century, a young man may have studied laws at university or not, then he was apprenticed (by mid-XVIIIth Century, the apprentee would pay about 300 pounds) to a barrister who would teach him the job during seven years; or was supposed to do so, as some would send their apprentee to coffee houses while working at their office...
After the apprenticeship, the young man would become a clerk. An average salary for a clerk was, I think, about 50 pounds a year.
Then, if he had money and was agree, he could become an associate to the barrister, or in other cases succeed him.
The next step, to judge, I didn't see.