One folder per tape
The simplest rule is the best one: put each album in its own folder, and put all those folders inside one parent. Add the parent as a source folder in REAR and every album turns up as a separate cassette. The shelf is sorted by name, so names that sort well make for a shelf that reads well.
If you prefer to file things by artist, that works too. A folder that has album folders inside and no songs of its own counts as an artist folder. Its name becomes the artist on every tape inside it.
Loose songs dropped straight into a source folder end up on one tape called Singles. That's handy for odds and ends, but if you find yourself with a hundred of them, a few folders will serve you better.
A folder with 40 tracks from 12 different albums works, but it becomes one very long tape. If that's what you want, great. If not, split it up before you add it.
How the artist name is picked
Audionix counts the artist tags in a folder. If one artist appears on at least half the tracks, that name goes on the cassette. Otherwise it says Various Artists. So a soundtrack with one composer on most tracks reads as that composer, and a party mix reads as Various Artists.
Clean tags help here. Misspelled or blank artist fields can push a normal album into Various Artists. Audionix can look up missing names online (more on that in the cover art article), and you can always set the artist by hand.
Naming things
The cassette name starts from the folder name, so a folder called Summer Drive 2019 reads better on a label than New folder (3). It's a boring tip. It also makes the biggest visible difference.
Mixed formats are fine
Audionix plays MP3, FLAC, AAC/M4A, WAV, OGG and more, and a folder can hold a mix of them. The tape type follows the files, so a folder of 24-bit FLAC will look different on the shelf from a folder of MP3s. That's on purpose.
Once the folders are sorted, the coverflow is where you'll spend most of your time.