How does one get a D rank

Pretty darn bad. All of the grades are based on your percentile performance, so S+ grade means you need to be in the top 2% at one extreme, so D grades are probably similar with a D- probably being in the bottom 2%.

The bottom percentages of performance are probably filled with the performances of intentional feeders and similar, so even full on afk matches could potentially net you a C grade since you're at least not feeding. Any game (even one where you get utterly stomped) where you're actually trying is prone to be at least C material as a result, although of course the champions popularity and the role you're playing them in are going to be factors as well.

Youd have to int and even that might not be enough

There's a D rank?

Huh.

no wards no items no xp just feeding on one of the highest winrate champions

Sometimes I get D+... like 7-8% But I got D- without leaving and being AFK...