Select blocks from the toolbar and place them onto the grid to make your level. After making the level, click export. Double-click the string of numbers and copy it to your clipboard. Open the marioBasic engine (link below) in a new tab and see the instructions to continue to playing your level.
This is a tool to make levels for marioBasic, a Mario game engine on Scratch. marioBasic Engine (Level Player): https://scratch.mit.edu/projects/294098620/
