The secret is you don't.
After the game you're in now you'll never see them again, so why teach them? If you really wanna be kind maybe after the game add them and maybe discuss some better builds...But while in game there is almost nothing you can do. Aside from lightly suggesting a certain item.
But still there is no garrentee they'll listen.
I guess the best thing to do is watch what their building, and use that to build yourself accordingly.
Like for example you see your jungler unwisely building full damage and you're currently the top laner..Well, It wouldn't hurt to change up your build to be less damage/ more cc peel heavy.
And vice versa, if your jungler is building full tank, it wouldn't hurt for you to add in a damage item extra into your build to compensate.
I guess what i'm trying to say is, You can't control what they build...But after they build something you can then use the knowledge of how they're building to build yourself a better build for the current situation.