Why do my beats not line up with the song?

The BPM of your level is wrong. Even a small change of ±1 can make a huge difference. Sometimes songs have a decimal BPM, or the BPM of the song changes halfway through the level. If that's the case, place a new Set BPM event in the Sounds tab where the BPM changes.

I know my BPM is correct, but the song still doesn't line up?

In this case the song's Audio Offset is wrong, so return to your Play Song event and follow the instructions in the following article.

I've finished my level, how do I know if it's fair?

You can ask for help in the #level-editor channel in Discord or ping the Editor Helper role, a group of people who have signed up to help making levels.

I finished reading this guide but I still don't know where to start!

Play other people's levels and open them up in the editor to really study how they were put together. If making visuals is too much work, focus only on placing the beats, visuals are secondary to the gameplay.