google – Best way to move rankings from one domain to another

Domain X appears on the first page of Google with a specific search word, while site Y (my site) appears on the 5th page.

I was offered ownership of domain X. What is the best thing I can do to improve the ranking of site Y?

  1. Redirect pages from X to the appropriate page from Y?
  2. Put links in the pages on X to the relevant page on Y?
  3. Return something in a header?
  4. Something else?