Instead of `git add -p`, you would use `sl commit -i` (whose interface I much prefer). To amend into a previous commit, I prefer to switch to it and then just use `sl amend` (+ `sl restack` if necessary), but you can also use `sl fold` IIRC. Instead of `git rebase -i`, you can use `sl histedit` (not a direct replacement for autosquashing, but worth mentioning).

To split a single commit, you can use `sl split`, which is quite difficult in Git. (I miss that feature in Git quite a lot.) You can also use the `sl absorb` command to automagically merge local changes into the previous patches where they seem to belong (roughly speaking, commute changes backwards until they would cause a merge conflict, but it's a little smarter about avoiding certain merge conflicts).