It requires a different mindset.
Annie Lamott suggests taking a printed copy of your work to another room. Donate to an environmental organization if you feel bad about using so much paper, but we are writers, we need to use paper and lots of it. Go through your manuscript as if you were looking at a work from a fellow writer.
Be kind, but true.
Do not offer hard criticisms to yourself, but be real.
Suggest changes and find the greatness in your work.
Don't be afraid to let go.