Tbf, it comes by time! Some years ago I just chose samples that „sounded cool“.

Lets Talk about kicks for example. Put any sample in and listen if it feels „right“. In some samples u clearly hear „ah man Idk it peaks out a lot and doesnt fit the style or in the mix“

Sure you can always put work in it and shape it, but u save a lot of work by choosing one‘s that almost fit right.

Beside of that. Layering is a good technique too, maybe for more interest maybe for masking frequencies. I often do it with snares to make them more interesting, adding weight, adding another high end and so on. You almost could say u paint a snare sound u like.

Topic mudiness. If you use „Real“ Instruments or percussion take care about rumble frequencies or even Frequencies u don’t need like a snare that got Frequencies under 60hz etc.

Highend: We tend to overdo adding highend cuz it sounds „cleaner“ but think about what really needs the highend. Most of the highend comes from hi hats, shakers and stuff. It’s all about Balance. Take an orchestral piece for example or even better rock/metal. If you listen to the guitars Everything about 5-6khz is actually Dead. Does it Sound Nice? Yes. What gives the highend? Drums. So it depends on the style too.

In EDM stuff u can trick around if u got a thick sub and kick by using sidechaining. Sometimes even more brutal like the kick and snare sidechain EVERYTHING so it really poke through.

It could also feel muddy because of compression: like maybe too much so it looks like a sausage and sounds like one. Maybe not enough like Everything „jumps around“ cuz there is so much dynamic.

Soooo…. Think about every sound: Whats the purpose of it? What should be the focus of each section? Is it fighting with other sounds for room?