Booting up

Download the cartridge.
Put the cartridge into the folder containing your music carts. It's better to just place it into the root folder of the system so that Pictunes will be able to see any cartridge inside any folder.
Boot up the cartridge. Use the left and right D-pad arrows to navigate across the top bar. Select the D button (the hint at the top left should say "DIR") And press the O button to enter DIR mode.

DIR mode

In DIR mode, you can navigate the Present Working Directory of the Pictunes cartridge. Use the up and down D-pad arrows to navigate the folder. Subdirectories are pink. Press O to enter a subdirectory when it is selected. Press X to exit the subdirectory, Cartridges are gray. Press O to select a song from the cartridge when it is selected.
Choose the "LOAD ALL" list item and press O to queue up all cartridges in the directory (not recursive). You can use your folders as playlists!

Load subsong

In the subsong selection screen, you can either load all the soundtracks from the cartridge, or, using left and right D-pad arrows, choose a number for a specific song (you will have to know the number beforehand, as Pictunes doesn't know how many subsongs are in the cartridge that it hasn't yet loaded). Generally, loading all the subsongs is simpler. Press X to cancel, or O to confirm.

Queue

When you choose to load either all or just one subsong of a cartridge, on the left half of the screen you will be able to see the green text "CURRENT:" followed by the name of your chosen cartridge and the number of the selected subsong (or the letter A if you've chosen to load All subsongs). If you load an another song using the DIR menu, it will be queued (indicated by the yellow color).
Hit X several times to exit the DIR mode. Navigate to the Q button (the hint at the top left should say "QUEUE") and press O to enter QUEUE mode if there's any songs queued up.
The queue is split into two parts: the current song (green) and the queued songs (yellow). In QUEUE mode, use the up and down D-pad arrows to navigate the queue. Use left and right D-pad arrows to change the queued songs' order. Press the O button to remove a song from the queue (or to remove the current loaded song). Press X to exit QUEUE mode.

Playing the song

Navigate the top bar and choose the P button (the hint at the top left should say "PLAY"). Press O to start playing songs. When the song ends, it will automatically start playing the next song in the queue. The button will change to "STOP". Press it to stop playing songs.
If you chose to load ALL subsongs from a cartridge, the queue item will "unfold" upon loading the cartridge: the item "CARTNAME-A" will disappear and the individual found subsongs "CARTNAME-1", "CARTNAME-2" etc. will be queued up.
Notice! If the current song instantly disappears, it means that either the selected cartridge doesn't contain any music (if you chose to load ALL subsongs from that cartridge) or the specified subsong number is too big (the cartridge contains 2 soundtrack but you chose to load the 3rd one).

Skipping songs

To skip the currently playing song, navigate the top bar and choose the N button (the hint at the top left should say "NEXT"). Press O to play the next song.

Looping songs

By default, songs will loop when they encounter a loop point. To toggle this, navigate the top bar and choose the button L (the hint at the top left should say "LOOP"). Press O to toggle song looping.

Visualizers

To enable the visualizer, navigate the top bar and choose V (the hint at the top left should say "VISUALS"). Press O to enter the VISUALS mode. There, press O to change the built-in visualizers, or press X to exit VISUALS mode.

This is a mode-based tool. The main mode is free mode. You can always enter it by hitting BACKSPACE. Hit BACKSPACE again to return to previously used mode!

Here's a short guide to the modes. I will not list the keybinds here. Hit H to see the keybinds for the mode you're currently in.

To start drawing, hit O for entry mode. It gives you access to the full palette of P8SCII characters to choose from.

If you prefer a text-editor-like workflow, hit I for insert mode. It allows you to directly enter characters from your keyboard. To return to free mode, hit BACKSPACE. By default, the cursor does not automatically advance when a character is entered. To enable this, hit TAB anywhen. This feature is indicated by the "--" or "->" to the right of the mode's title in the header bar.

Hit S to enter selection mode. Upon entering, your current cursor position will be marked as pos1. Move the cursor to choose the pos2. Then hit ENTER, C, X, or D to respectively CONFIRM your selection, COPY it right away, CUT it or CANCEL it. You can also hit S again to reset your pos1.

When selection is confirmed, it is active and certain actions can be applied to it to alter every character within it at the same time, using entry, color or decoration modes.

When selection is copied or cut, you will automatically enter paste mode.

To apply color, hit G for color mode.

To apply decoration characters (sort of a second layer), hit F for decorate mode. Decoration characters do look buggy when there's per-character background colors enabled.

To turn off per-character background colors, per-character foreground colors, or decoration characters, open the pause menu by pressing SHIFT + P, or holding down the pause button. There you will find the options to disable these features and to choose a dummy background color that will not be saved in a produced string.

To save your drawing to the clipboard, hit W.

To load a drawing from the clipboard, paste it into the PICO-8 system by pressing CTRL + V and hit Q.