Starting out:
- Make sure your timing is accurate, otherwise it can be hard to get the rhythm correct. If you already know how to time songs, great! You can ignore this point. You may be able to get accurate timing from another mapset of the same song, but if there are none or the timing is wrong, you're going to have to time it yourself.
These guides can help.
- Learn to use the editor's shortcuts, they can improve your efficiency by a lot. The keyboard shortcuts are shown next to their respective functions in the editor's dropdown menus.
- A lot of people will tell you to start out with short and simple anime songs, but if those aren't your style, there's no point in that. Mapping music you like is most likely going to be a lot more fun. Granted, it is easier to learn the basics when the song's rhythm is simpler, but mapping more complex songs right off the bat isn't going to kill you. Sure, the maps will suck more, but you'll get there eventually.
Moving forward:
- Get feedback from others. It's often hard to see your own mistakes, so other perspectives are valuable.
- Don't get stuck on one map. If someone tells you something about your map sucks and how to make it better, just keeping that in mind when making your next map is more productive than fiddling with the old one. The exception is if you're trying to rank the map, but there's no point in doing that until you're more experienced.
good luck