Set the scale and resolution then click the flag or press space to draw.
EDIT: I fixed the error! And I added a resolution slider. Based off of https://thebookofshaders.com/edit.php#11/2d-gnoise.frag (ported from GLSL to Scratch) I think I got something slightly wrong since there seem to be hard edges here, but it's at least mostly working as expected. If someone can figure out where the error is I'll fix it, but otherwise I'm too lazy to find it myself. :P