Click on the canvas to add control points. Press space to finish the curve. Then, you can click and drag the the control points to edit the curve. But, if you click on the blank canvass it will start a new curve. If you like this project, suggest it to be featured here: https://scratch.mit.edu/discuss/topic/2733/
I used this site, http://www.cs.mtu.edu/~shene/COURSES/cs3621/NOTES/spline/Bezier/bezier-construct.html, for the equation of the curve. I used Sheep_Tester's combination custom block, which can be found here. https://scratch.mit.edu/projects/81627406/#player All other scripts are my own.