1) Install Pressflow
This should be relatively seamless; don't be afraid just to replace your entire Drupal core with Pressflow and simply drop your sites/ directory in. The extra prudent might run Hacked just to make sure core hasn't been altered, but I say move fast and break things.
2) Install Varnish
There are relatively comprehensive instructions for installing the basic Varnish service on all the relevant Linux flavors here.
3) Configure Varnish
The Four Kitchens wiki has a good starter config setup; you'll probably be placing this in /etc/varnish/default.vcl
4) Connect Varnish with Drupal
Install the Varnish module and insert your Varnish Control Key (should be in /etc/varnish/secret) in the module config page (admin/settings/varnish/general).
Additional Resources