Click and drag (or tap and drag) to place blocks. The bar at the top is where you can select what block to use. Hover over/tap on a block to see its name and description, then click to select the block if you want it in your level (if you're on mobile you've already selected that block when you tapped it for its description).
Use the inputs or arrow keys to resize the level. Increasing the size of the level will use the current selected block for the newly created blocks.
There are also undo/redo buttons; I hope you know what they do.
If you're on a Mac you still need to use the control key for the keyboard shortcuts.
up arrow increases the height of the level
down arrow decreases the height of the level
left arrow decreases the width of the level
right arrow increases the width of the level
ctrl+shift+Z or ctrl+Y redo
ctrl+enter test the level
c equip the block picker tool
play_circle_filled test the level in the player.
save get the level code or import someone else's or save it to the cookie where the level is kept.
list load an example level if you want a base to work off of.
playlist_add submit a level for me to add to the example levels or a level-bit for the random level generator.
title change what the text blocks display.
help looks like you've discovered me!
more_horiz links to other uninteresting places.
If you don't know how to close this help window, click that red dot in the top-right corner of the pop-up dialogue. :P