You do realize that this represents many hours of language study, don’t you? If you insist on a simple answer, then here you go: Simply use the simple past (unless it was an activity in progress, then use the past progressive). The simple past is the most common way of talking about the past, and you will rarely go wrong. Even though you’re talking about today, you’re thinking of what you did *earlier* today, aren’t you? This morning, two hours ago, whatever — that’s the past — if you state (or think) the specific past time, you cannot use the present perfect. If however, you are thinking of the events from earlier today as part of a “today” that is still going on, or if there is a connection to the present (“I haven’t eaten today, so I’m hungry now”), then it’s more like the present perfect — but in colloquial American English, we can still get away with using the simple past. Important: For an action (dynamic verb) in the past that you are presenting as ongoing when something else happened, we use the past continuous (also known as the past progressive). This should be easy for you, as the criteria are the same as for when we use the present progressive. If you haven’t mastered the present progressive, then do that now.
By the way, Alex, feel free to share your thoughts on my discussion thread!