one really good tip i can give you (apart from reusing things to create structure as i say below) is to reuse placement! if you've put down a few notes, and want to know where to place the next note, put it where one of those notes were! Check some of my maps or relatively any good quality map to see this (and the reusing notes to create structure) method used almost 100% of the time.

i suggest not spending too much time on maps early on. every map you make will be a significant improvement, but if you stay behind on older maps they can and will stunt your improvement and in some cases actually be detrimental to your skill overall. until you're at a point when you're relatively problem-free (you're looking at six months to a year on average for that to happen, so patience is key) you should just stick to making a full set (or if you want to focus on something in particular, just an easy/normal for example is fine), getting one or two mods, and moving on. you can do whatever you want, i'm not forcing you to do this, but i am letting you know that it is definitely the best way to improve at mapping, and sticking to maps too long will be detrimental to your improvement and your current skill. this is something you should definitely want to do, but again it's your choice boyo.

make sure you consistently check to see if something flows right. If it's comfortable for you, it's comfortable for the player (most of the time). you need to watch out though, because the more you play your own map you'll get used to bad flow and won't notice it, so make sure you only check once or twice! you don't even need to go into test mode, just play the song in editor and glide your cursor through parts of the map to check them. Also, if you're not already, don't use grid snap, it really limits what you can do creatively.

Reusing (copying and pasting things you've used before in a map) may seem like a lazy way to map, but it actually adds some really nice structure to your map and makes it much better looking overall. Check any good map and you'll see what I mean, it's used often to create symmetrical patterns and just patterns in general.

try to have consistent spacing! the only reason you should change the spacing is for emphasis. if something is quiet and soft, lower the spacing. if something is loud and heavy, make the spacing higher. You should try to find a consistent distance snap to use throughout the whole diff. You don't even need to use distance snap, just use it as a guideline for what to do. check the upper right corner when you have a note selected to see the spacing between the note ahead of it and the note behind it.