Hi Vasilis,
Sorry that I didn't get back to you earlier.
We do have a few videos on HUDs, but they all seem to require a higher status. W34z3l made a video about that last year during his 6-max walkthrough, and he invited his viewers to post screenshots of their HUDs in the discussion thread (starting around here), so maybe you can take a look around and see if you can find something useful.
The most basic values that virtually any HUD contains are VPIP and PFR, which describe your opponent's preflop behavior. Other basic values include their ATS (attempts to steal) and Fold vs 3-Bet, which are useful when you're contemplating a resteal or blind defense, and their Fold to Steal and 3-Bet%, which are useful if you're considering stealing yourself. My own HUD also includes mouseover stats about their raising/stealing behavior from any particular position. It may also be useful to include stats about 4-betting or their Fold to 4-bet%, if you're facing tricky opponents.
The most basic postflop stats include their C-betting behavior and their Fold to 3-bet%, but my own HUD actually includes these stats for all three streets. A more basic way of including information about their behavior on later streets would be their WTSD% (went to showdown). It is debatable whether or not the W$SD% (won money at showdown) is useful, I personally don't include it. If you want to add some more advanced stats, you can add some about their donkbetting, their likelihood to float and their behavior out of position.
Obviously, it is also important to include information about how many hands are recorded, otherwise you'd have no idea how reliable your stats are.
I personally don't use my tracking software before the beginning of a session, I'm not aware of any preparation techniques. When it comes to session reviews, you should basically take a look at any hand that made you wonder if you played correctly, and then post it for discussion if you're still not sure. A common advice is to focus on big winning and big losing hands, but I have heard that it might actually make more sense to look at more "typical" hands, that describe your more basic play. So it might also be a good idea to post some hands for discussion or review if you think you did everything correctly, just to see if there are any leaks in your typical play.
I hope this was helpful, but feel free to ask if you have any further questions.
