How do I squash commits together?

A few commits on your branch: open Repository ▸ Edit History… (⌥⌘E), select a commit and press ⌘J to combine it with the one below, or drag one commit onto another. Edit the combined message in the inspector, then Apply. Commits made with git commit --fixup arrive already combined. Editing history has the rest.

The whole branch, as it lands: a squash merge puts the branch’s changes on the target as one commit and leaves the branch’s own history alone.

If the commits have been pushed, combining them rewrites them, and sending the result needs a force push with lease.