There is a great article on how to use the table extension framework in AX 2012.
Keep in mind that if the parent table's primary key is not RecId-based, but the table has CreateRecIdIndex property set to Yes, then the relation on step #1 should be created based on "Single field AlternateKey based"; otherwise the new feld will potentially have wrong type, and it will not be possible to use it in the SysExtensionSerializerExtensionMap map.
Keep in mind that if the parent table's primary key is not RecId-based, but the table has CreateRecIdIndex property set to Yes, then the relation on step #1 should be created based on "Single field AlternateKey based"; otherwise the new feld will potentially have wrong type, and it will not be possible to use it in the SysExtensionSerializerExtensionMap map.