To answer the question posed directly: you don't.

To elaborate—

There are a couple of really important human being things that many MMOs players don't seem to understand. First, unsolicited advice — no matter how well intentioned or polite it appears — is considered rude behaviour in civil society. Almost every single person who has ever received unsolicited advice recoils at it, most often with a, 'Who does this guy / gal think he / she is telling me what to do!' There are varying levels of response, and some people may take it well, but no one should be surprised when unsolicited advice goes over poorly. Secondly, the only time people learn is when they have made the active decision to learn. These two points are linked together. So, even if your unsolicited advice goes over well (it usually does not), if the person does not have the desire to learn or improve, you're wasting your time. Which is why random groups in North American culture are such a mixed bag. Most people find unsolicited advice rude, and the very few who do not, need to have the desire to learn in order for that input to have any effect (and quite honestly, the lion's share of people who have the desire to learn will just Google how to play better).

So what do you do? As I said, you don't inform people they're playing badly, and you certainly don't offer unsolicited advice on how to improve because of the above. If their play is preventing you from completing your objective, you leave the group and find a new one. Fortunately in this game, most of the content can be done with nearly everyone playing poorly. But, honestly, trying to get people to improve whom you do not know (and so that unsolicited advice will always come across as rude) and likely don't even have a desire to play better, it's a losing battle. We really need to stop doing it. Dating back to the early MMOs, there is so much evidence of this not working. Just find new groups or play solely with people you know. It's the only way out of this rut.