I believe you are referring to the gateways, not the seals of the Nethers themselves. In the Book of Azazel, EA talks about placing the gateways, which represent the Houses, on the walls of the Temple.
The first gateway is the Gateway of Pacts, to be draw around the Operator, or used in the place of a Triangle of Manifestation.
The second gateway is to be placed upon any object as a way to consecrate it and connect it to Azazel’s legions.
The third gateway is for the House Anatel, the Demonic Warriors. This is the House to which Salas’ash and Suhn’tal’ock belong. It is to be drawn on a wall.
The fourth gateway is for the House Retztael, the Demonic Priests. This is the House to which Ant’harratu and Fro’ghla’'tasch belong. It is drawn on a wall.
The fifth gateway is for the House Malkash, the Demonic Informers. This is the house to which Mammy’aon and Tah’ka’yat belong. Also to be drawn on a wall.
When you summon a Nether from one of the Houses, they will enter through the appropriate gateway (I’ve never used the gateways but I assume they would make it easier for the Nethers to materialize though)
Ant’harratu appeared to me pretty much like his description in the book, a black hooded figure.