Click on any of the 3D grids to place the cursor there from the perspective of the grid. Press V to place a vertice at the 3D cursor. Press E when the cursor is on one vertice and again when the cursor is on the other to place an edge. Press R to delete all vertices and edges. Use the arrow keys to navigate the panel your mouse is over. To move the 3D camera, use WASD to move and the arrow keys to change direction. Press space to go up and Z to go down. Put your mouse in the top left corner to change some options. Press M to toggle between edit and view modes. Press P to generate the code for the world. Press L to load one. The top left panel is the 3D perspective. The top right panel is from the top perspective. The bottom right panel is from the side perspective. The bottom left panel is from the front perspective.
This is inspired by the Hammer World Editor used by the Source Engine. A BIG thank you to @WO997 for helping me make the 3D rendering. Post some cool worlds in this discussion! I might feature some! https://scratch.mit.edu/discuss/topic/219054/?page=1#post-2218292 You can also remix this if you want to.