Logging an activity is simple!
Click on the prospect profile that you want to log an activity for (as shown below)
Then click on the log activity button from the bottom right corner of the prospect page (as shown below)
This will prompt the log activity window to open.
From this window you can choose what kind of activity you want to log. Once you have chosen the activity, simply click the log activity button and you're good to go!
You can also log an activity by moving them from one stage to another, you can read about that here: