Technically, there currently is no way to do this. The listing of steps is more for helping you plan then for actually managing the process. There are a multitude of productivity tools that will be able to do that much better than we will.
All that being said, if you want to try and keep track of it, you do have one option. Since the list is plain markdown, you can add a strike through to show that it's done. Click edit on the goal and on the step you've completed you can add ~~ around the line.
For example:
- ~~Do this thing~~
When you save it, it should have a line through it.
Do this thing