Prior to filling in the Deduction Board for Polly Nichols' murder you want to ensure that you find the following clues:
Inspect the chalk outline and use the magnifying glass on the following:
- pool of blood
- neck (cuts)
- tongue
- bruised cheeks (both)
Clues not on the body:
- street sign (end of the alley)
- street lamp (end of the alley)
- the lady and man (in the alley)
Now to proceed to the Deduction Board (press "Y" to enter the sub-screen, and go to "Boards"). If you are having trouble filling in the board, you can use the following guide provided by simalcrum over at IGN:
http://faqs.ign.com/articles/108/1087365p2.html