Location

The first time Inutaishou is mentioned is because Sesshoumaru is searching for his grave.

In an addition to the anime, the Nintoujou, or staff of heads, first leads Sesshoumaru and Jaken to a tomb that is guarded by wild dogs. Sesshoumaru makes short work of the animals, but the Nintoujou proves it to be empty.

In both the anime and the manga, Myouga departs to find Inuyasha and admits that no one knows where his father's remains are. It is later revealed that they are in the youkai graveyard, and the means of getting there is the black pearl inside Inuyasha's eye.

Outside

Inutaishou's grave is a skeleton that is wearing armor and appears to be sitting up.

There is a hole in his armor to the bottom right, that matches the wound you see on his true form during the third movie. It is also the spot Housenki occupies during the second trip to the grave during episodes 155-157.

His left fang has been broken off near the base. I would say it looks like what was used to pin Ryukossei, butt the manga and anime are very clear that he used a claw. It is most likely the fang that was used to make Tessaiga and Tenseiga, though why he didn't remove the whole thing and have it regrow is unclear.

Inside

Tessaiga was stuck into a platform against Inutaishou's spine, and only a human (Kagome as was the case) was able to remove it. Exactly why there are so many skulls and bones in his ribcage remains unknown, though the majority of them do not look humanoid.

The fight between Inuyasha and Sesshoumaru in his true form takes place inside Inutaishou's midsection in the manga and on his right shoulder in the anime. The reason Sesshoumaru turns out to be just big enough to fit in his father's ribcage and Inutaishou's dog form is so much smaller in the third movie may be because they are able to control the size of their dog forms.

Gateway

The first time the grave is visited, everyone uses the black pearl that Sesshoumaru ripped out of Inuyasha's eye. Sesshoumaru uses Nintoujou to open it up and he, Jaken, Inuyasha, Kagome, and Myouga all enter the gateway.

When the group goes back to fight over the final shard, Naraku kills Abi-hime's mother. Because she is related to the skeletal birds that live in the youkai graveyard, her blood serves as a passageway between the two worlds. Naraku enters, and Inuyasha, Kagome, Miroku, Sango, Shippou, and Myouga follow after him.

Sesshoumaru comes across the body of Abi's mother, but he's already to late to cross through. Kagura shows up and leads Sesshoumaru to the mountain where Gozu and Mezu guard the gateway to the youkai graveyard. He uses Tenseiga to pass by the two guards, and Jaken follows him through.