I'm pretty sure that you only have to attach the top half of the page on the right (Page 1) to the package (glue or clear tape so everything is clearly visible).
Page 2 (customs declaration) is identical to Page 1 (except it doesn't contain bar code postal info), and any mail I have ever sent/received by mail just used the info from page 1 as the Customs Declaration (actually it just used a separate Customs Declaration form from the post office).
Page 3 is for the shipper/dispatcher. Fine if you are using a courier, but not needed for postal packages.
Page 4 is for your records (or for your recycle bin).
That form is designed for couriers, where they submit actual paper documents to customs. USPS packages are processed differently at the destination country, so the declaration attached to the package is sufficient.
FWIW, USPS has a much simpler Customs Declaration form, but then you have to put the address information on the package separately.