Currently exporting doesn't work in browser, so I would suggest running directly in Pico8. P.s. If there are any bugs or errors, please let me know! :)
As of 0.2.4, you can now add Custom One-Off Characters! This tool, allows you to easily make and export, your custom characters without the hassle of doing it by hand. Currently you can export the characters in both Hex and Binary Formats, but the tool only accepts the Hex Format or GFX directly from your game.
How To Use
In the black grid, left click with the mouse to draw white squares and right click with the mouse to draw black squares. To reset the grid, left click the bin icon.To change the colour of the Char, left click your intended colour from, the colour bar.
How To Import
You can import by, copying the One-Off Characters(2 Digit Hexadecimal Format) or by copying a sprite directly from your game in the GFX format(Non Black and White Sprites get mixed results). Then paste it into the Cart, and a preview on the left should appear. To load it into the grid, left click the import icon(The one above the upper Copy Icon on the left).
How To Export
It's as easy as left clicking one the Copy Icons(Bottom Left Buttons), then pasting it into your code. Click the upper one if you want it exported in the 2 Digit Hex Format, or the lower one for the Raw Binary.