As a new modder, most look at maps of those who are fairly experienced, and they have absolutely no clue on what to actually say.
Most of them usually don't reach past the Ranking Criteria as it isn't easy to digest right away. So with all that being said, this section within the guide will mostly focus on the osu!standard Ranking Criteria and explain most of what is said in a simplified manner.
Standard Ranking Criteria
Link: https://osu.ppy.sh/help/wiki/Ranking_Criteria/osu!
Upon looking at this and you are possibly a new modder, you will likely get confused upon reading most of these heads on and will likely struggle how to go about it right away so here is a more simplification of what is being said within each section as well as a tip how to go about it.
Anything that is a Slider or Circle, must not go off-screen at 4:3 ratio.
Can always check this at 800x600 resolution and testplaying while pressing P (for Pause) to get an overall look at it.
Spinners must be long enough for Auto to achieve a 1000 bonus score.
Always play in Auto to be sure that is the case before posting about it.
Beatmaps must use min. 2 different custom combo colors unless default skin is forced and shouldn't blend with mapset's background/storyboard video.
For Forced default skin checking:
Go to Song Setup
Go to Designs
Go look under preferred skin and see if it says default, and if not then make a post about mapper to change combo colours.
For whether the combo colours blend: Use common sense to judge this.
Every slider should have a clear path of what is being followed upon playing.
As you can see with this skin that I'm using in the picture, it becomes more unclear which path I'm supposed to in the get go as it starts at the very beginning. If you see this then feel free to post about it and bring up your concerns to the mapper.
Otherwise more ambiguous sliders should be judged on a case by case basis and depending on difficulty as a whole.
Do not use keysounds without hitnormal support. Players do not receive enough feedback otherwise.
Self-Explanatory, if you listen to a map either at 80% volume and 100% hitsound volume and you barely hear the hit normal but loud but the mapset itself is keysounded, then it is a major issue.
If you are unsure if a mapset is keysounded turn music volume down to either 0 or 10% while effect volume is much higher to hear.
If a hitsound is used as hitnormal, ensure it does not blend with the song. Players do not receive enough feedback otherwise.
Self-Explanatory and more or less similar steps to key sounding upon how to tell if it blends.
Difficulties must convert to other game modes without breaking star rating/performance points.
A completely rare occurrence that nobody really has to worry about. If you are concerned about whether this is a problem within a mapset, simply change the gamemode type in the osu! selection-screen and see if you notice anything odd in the sr.
Usually it is fairly obvious when it is problematic or if it has a reason to be like that.