Time to pull out my most popular YouTube short:
Basic Vocaloid Mixing Tutorial
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Mixing is always subjective, and you just use your own taste, but it's a core part of music production. The better you get at it, the more you can let the musical parts shine.
You can break mixing into three essentials: leveling, processing and corrective surgery.
Leveling is literally just setting volume levels. Some things should sound louder than others...like you obviously don't want your cymbals to be louder than your kick drum and vocals. And this might be something you just do as you make the music, not something you spend time on later. Your goal, by the time you're done, is to have it sound cohesive...and not be clipping on the master bus. You want hitting close to zero but not going over. (I'm going to omit the topic of master compression and perceived loudness, because that's more complicated.)
A good way to achieve this is to get your kick drum peaking at -12 and bring up the volume of other parts until they sound good relative to that kick drum. Using that reference will help you build a reasonably balanced mix. e.g. you probably instinctively know you want your kick to be a little louder than your vocal, the cymbals quieter, and your bass instrument to not drown out the kick, etc.
Another tip is to set the gain/volume control in instruments to roughly where you want and use the fader on the mixer board for fine tuning. The faders use an exponential scale, so each notch is a bigger difference in volume than the last, which makes it hard to dial in small adjustments.
Effects: know what reverb is and play with it. It's your friend, and will make even simple sounds sound more impressive. It gives them presence. Pretty much every room you've ever been in, unless it's been treated for recording, has its own flavor it imparts on sound. You hear the small reflections of the sound off walls and ceilings, and your brain makes assumptions about the space based on how it sounds. Your virtual instruments, including vocals, will sound "dry" and artificial without that sense of space. So if you add reverb and dial it in right, you create a sense of all of the sound being in a real space. It's a night and day difference.
There are other effects, like distortion/saturation, delay (echo), etc.. But start with reverb.
Equalizers and compressors are what you could call "corrective." Learning when they can improve things is important, but equally important is not using them without a need. They're much more complicated to learn, but well worth it.
Compressors will give vocals more presence and strength, squashing the loud and quiet parts of the waveform together to make them more consistently loud.
EQs are just are just a fancy volume control that lets you target ranges of frequencies. Like you can cut the low part of your bass so it doesn't step on your kick drum, or cut the low part of cymbals you don't care about to carve out space so your piano can be a little more present.
Combining all of these is especially important for vocals. Vocaloid is just like human recordings: singing sounds flat when you record it, and mixing breathes life into it.