This one, which is called "Flower Note" in your inventory, is on the table after you wake up after you explore one of the hidden areas in the cellar --

The Cellar

-- behind the rack of bottles.

Behind the Bottles

This particular bit of doggerel won't be of any use to you until you get into the crypt.

The Crypt

You can ignore the first two lines; they're just for atmosphere.

As for the rest of it, you can follow that part pretty much directly, with some allowance for poetic imagery.

For instance, when it tells you to "Gather your dried petals where immortality resides," there are some dried-out flowers inside the crypt. You can't take just the petals, so take the whole flowers.

Next, it says, "Place a flower where she stands before the stone turns to dust." Outside the crypt, there's a statue of a woman with her hand held out. Go put the flowers in her hand.

Nothing seems to happen except she drops the flowers and you can't put them back. Now what?

The last line says, "Away and back to earn her trust." So back away from the statue and go back inside the crypt, then come back out.

This time, look very closely at the pedestal the statue is on, and you'll see a piece of paper that wasn't there before. (You can only see it when you're on the path from the crypt.) That piece of paper --

-- is the design for one of the coffins in the crypt, and the drawing shows you which coffin.

Flower Coffin Design