First open the urn - you'll get a map of the palace. Then save your progress - the game will suggest you to save on a new file because you won't be able to return to Rabanastre for some time.
Now go to the room for servants and run toward the stairs. On your way there pick every treasure you can find in this hall.
You'll get to know that servants aren't allowed to go upstairs, but after the cut-scene try even so. A brown seeq will see your attempt. Go talk to him. He'll say that he's willing to help. Use the square button to call the guard and go a bit aside [screen 1] and when the guard starts talking with the seeq slowly go upstairs [screen 2].
"Hawk Signet"... unfortunately isn't working...
...but the "Lion Signet" is a whole different matter.
Here you'll have to use your head, but don't worry - if any of the guard spots you you'll start from this point. Here I have attributed numbers to horizontal alleys starting from the bottom. Also I refer to the left vertical alley as "A" and to the right one as "B".
So let's get down to business: Run into the second alley and call the guard from the other side (to call a guard press the square button, but don't wait for them just run). Get back to "A" alley and run to "B" alley through the first alley. The guards standing along the "B" alley won't do you any harm as long as you don't try to run into the alley they're just guarding. In the "B" alley run to the third alley and use the Cresent Stone on the Lion Signet on the floor [screen 2]. Get back to the "B" alley and run to the fourth alley, call the guards, get back to the "B" alley, run to the third alley, call the guards, get back to the "B" alley, run to the second alley, call the guard and run to the "B" alley. Now run to the "A" alley through the first alley. Now run along the "A" alley to the fourth alley but instead of turning right turn left.
Approach the wall at the end (you can see a greenish light behind the grating) [screen 1]. Inside this corridor run to the end and press the switch on the left side. Move a bit back and pick the other branch of this corridor. Approach the wall at the end and pick the option: "Examine the wall" [screen 2].
After the cut-scene run upstairs.