I'm not really sure how to explain this, so i'll try my best 

You know how Battle Master gets multiple maneuvers and at level 3, they get a limited amount of those maneuvers. And as they level up, they gain more battle maneuvers. How would I go about doing that for a homebrew subclass?

Others can probably explain this better than I, but when you create a feature, below the description check the Has Options box and next to that list the levels each of those options are gained. For instance, Battlemaster would look like:    3,3,3,7,7,10,10,15,15    and grant you three options at level 3, and two additional options at levels 7, 10, and 15.

Then you add those options under this feature as Class Feature Options. Each of those can have level prerequisites if some of your options require higher levels, like how Eldritch Invocations work.

As in similar to the trickshots of gunslingers? As scatterbraind explained, in the subclass, where there is a entry called "Levels where options known" type in the levels based on how many options when, for example, "1,1,7,7,15,15" would allow the character to select the options created twice at level 1, twice at level 7, and twice at level 15. If you want anoption to only be available at a certain level, when creating the option there is an entry for minimum level, so if you want one option to be available at level 7 only and not one, do that

I gave an in-depth tutorial on Options here; (Subclasses & Races #1): https://www.dndbeyond.com/forums/dungeons-dragons-discussion/homebrew-house-rules/131411-a-homebrewers-how-to-faq