1) Hands that you have marked during a session. You should mark all the hands that cause some form of confusion or stress.

2) Biggest losing pots. Most microstakes players have pretty massive leaks on big pots on the river, so identifying and fixing those leaks is one of the quickest ways to improve winrate.

I would watch as much rio videos as possible to get a basic idea iso wide with okayish hands and extract value good 3 bet is 8-11% people at micros play terrible in 3 bet pots that where most of ur edge as aspiring pro comes knowing when to trap when to value bet when make pot odds call and include random idiot factor if u know opponent to make those call on river. Micros is all about discipline and experience getting Ur intuition right so u need volume then its just easy print from there and time to move up. some good tips to alwys play afternoon/nights/late nights and weekends same time. im saying that cause i never reviewed my hands until i got until 50nl but i just sucked up as much idea from utube videos and upswing couple months ago and never looked back but if u stuck at certain limit its definatly cause u have leaks so reviewing hands will defiantly help but watching videos teaches you more then reviews and nothing beats expierience backed up with good videos to bounce ideas from, I know it not exactly was ur question but decided to add 2 cent GL!

I'm not suggesting to mark too many hands or try to review daily a lot of hands. Today I did that mistake, have many hands to review but quality aren't good (have less time to go over them) as if I marked less.

I'm currently using two different tags for marking hands during play. One is just "Review" and the other "Priority Review". Like that, I try to limit the number of hands I want to review sometime before I play the next session while still not "loosing" the majority of interesting hands.

The normal tag "review" comes into play when I'm trying to study specific spots. For instance, when I filter for 3 bet pots played OOP as defender I then have 40-50 marked hands from the last couple weeks/months and start reviewing these instead of browsing through many random no-brainer HHs.

Depends how do you plan to work on your game, your current winrate, stakes etc

Problem with marking hands is that there is lot of different spots and you end up working little on 3bpot ip, EP vs BB srp ...
That dilute quality of improving and at the end you probable forgot most of stuff.

From my experience, and looks like your post is just at right timing as I just went into studying mode again, most effective way is:

BEFORE GRIND (take some time off just for study)

1. Choose spot (one you feel you have problem/uncomfortable or do DB review and see where your winrate looks bad, let's say SB vs BB srp).

2. Do some basic equity sharing for ranges on couple of boards (Axx rainbow, 986ss, KQ2ss...) using PIO/CREV/Flopzilla

3. Make some basic strategies for flop (example: CB whole range small size on Axx , check whole range on 986ss etc, polarized betting KQ2ss...)

4. Check population tendencies and see if some adjustment is needed to 3.

WHILE GRINDING (this is time where focus is mostly at applying premade strategies)

1. Focus on spot (play other spots as usually)

2. Review all hands after session from that spot only (filter)

3. Adjust strategies (if you're not sure check what coaches do in video, post hh...) - this could be added to BEFORE GRIND as part of #4.

AFTER GRIND (after 20-30k sample do some review)

1. Check if winrate improved for that spot. Variance factor is huge you can see if bottom of your open range winrate improved or check EV or just do another 20-30k hands if you think sample in unreliable.

2. Decide if you 're happy with this spot play or want to do more work (change basic strategies, focus more on turn and river play).

As you notice we don't mark hands. If we usually note a ton of hands that mean we have a lot of work to do so better to make some structure.
"Impactfull studying " from Paul Atwal is great video to check out for more info.