don't - Oxbridge have this silly, snobby system whereby your BA/BSc/whatever undergraduate degree is converted into an honorary masters seven years after you matriculate, and providing you didn't do something to 'bring the university into disrepute' in the intervening time. Which seems to mean, you stayed out of prison and didn't get conspicuously sacked from a posh job.
So far as I know, it's because back in the day, Oxbridge didn't like the idea of postgrad degrees at all, and so when they started coming in, Oxbridge just said ah, fuck it, we'll give them to everyone. Then, of course, they also started offering proper Masters courses that require actual study, but these have different names (Mphil, MSt, etc.).
If you are a reasonably bright graduate, you would expect to get a job somewhere where they'd check, and realize this thing on your CV called 'MA Cantab' means nothing more than 'BA Cantab', so it doesn't help you get ahead. And lots of people just leave it off, or never go back to get it.
It is, however, obviously the easiest way to get a MA without working hard!