You'll need some construction paper, scissors, glue, and a print-off of some piano keys.
To do this print-off, do a google image search for free piano key clip art. Copy and paste that picture into a Word/Google document and adjust the size accordingly. Copy and paste it across each line. One full page should be plenty for one child. I used 1 1/2 pages for my two.
Cut the piano keys out into long and small strips.
Next, cut out a big letter P. You can cut it out for younger kiddos, or let your older, more scissor-savvy ones do it on their own. We chose "pink" for letter P too, although you won't see any pink anywhere else in our house...
From here, they could do it on their own, really. But, I was the photographer, so I stayed around.
Have child line the P with glue from glue stick and paste the piano keys around, using the smaller ones for the curve in the P.
Affix onto a backdrop if you'd like and hang proudly in your home!
....or just act silly when mommy asks you to hold it up....
A good idea also is to have some piano music playing while this project is being worked on. You can look on itunes or YouTube, Pandora, etc.
Some recommendations: Mozart Piano Sonatas, Chopin Waltzes, Beethoven Piano Concertos or Sonatas.