Here is a very simple Pattern level editor.
Open pattern-level-editor.html in your browser, select a size, press
“New”, and then start editing your level. Click on a cell to fill it, and click
again to unfill it. When you’re done, click on “Export”, and copy the selected
string. This is your level ID. In Pattern, go in the “Game” menu, click on
“Specific…”, then enter your level ID, and enjoy!