You have a branch `feature_x` here. You've already pushed `d7322aa` with the proof of concept of the new feature X. After that you've been working to add new element to the feature, including some changes in CSS. Now, you want to squash your last three commits in one to make your history look pretty.
Now you can tell git what to do with each commit. Let's keep the commit `f392171`, the one were we added our feature. We'll squash the following two commits into the first one - leaving us with one clean commit with features X in it, including the added element and CSS.