Squash my last X commits together using Git

Use git rebase -i <after-this-commit> and replace "pick" on the second and subsequent commits with "squash" or "fixup", as described in the manual.

In this example, <after-this-commit> is either the SHA1 hash or the relative location from the HEAD of the current branch from which commits are analyzed for the rebase command. For example, if the user wishes to view 5 commits from the current HEAD in the past the command is git rebase -i HEAD~5.

You can do this fairly easily without git rebase or git merge --squash. In this example, we'll squash the last 3 commits.

If you want to write the new commit message from scratch, this suffices:

git reset --soft HEAD~3 &&
git commit

If you want to start editing the new commit message with a concatenation of the existing commit messages (i.e. similar to what a pick/squash/squash/…/squash git rebase -i instruction list would start you with), then you need to extract those messages and pass them to git commit:

git reset --soft HEAD~3 && 
git commit --edit -m"$(git log --format=%B --reverse HEAD..HEAD@{1})"

Both of those methods squash the last three commits into a single new commit in the same way. The soft reset just re-points HEAD to the last commit that you do not want to squash. Neither the index nor the working tree are touched by the soft reset, leaving the index in the desired state for your new commit (i.e. it already has all the changes from the commits that you are about to “throw away”).