When you mod a map, you point out the flaws or things you find that needs an change, in order to improve the map.
- Flaws are sometimes rule-breaking. You can check the Ranking Criteria to see if the map fits all of the rules. In the last section of the page there is the Guidelines. They are not obliged to be followed, but they are recommend and should questioned, whether this makes sense or not.
Flaws can also be something that doesn't make sense. For example, in the difficulty Easy, you see two objects (circles, sliders) with a huge spacing in-between, but another set of two objects with the same rhythm has a smaller spacing than the previous. This is a flaw in the difficulty Easy because the spacing of both sets are not consistent, therefore it should be point out. - Things you find that needs an improvement can be also flaws, but they can also be subjective. For example, when you think the pattern or the shape should be worked like that, the flow does not play very well, etc.
When modding, point out the objects or the timestamps first, explain why it needs to be changed, followed by a suggestion (like a screenshot). This helps a lot for the mapper to look what they did wrong and to avoid them for future maps. Though the mapper has the right to refuse your point, but they should explain why they didn't apply. This helps you see what was the mapper's intentions and learn new stuffs with mapping and modding.
Since this is your first time with modding, I suggest you get a mod, preferably from a good modder (and not a modder that points out only coordinations or "NC" without proper explanation) to see how a mod works. After you have acknowledge that, you can try to M4M if you like.
Also I recommend you check
this guide compendium or ask a BN / QAT if you need further help.