Arrow keys/WASD to move Steve, Space bar/"X" to delete closest block to cross. Left mouse click to place block to cross. 1 to switch block to grass, 2 to switch to dirt and 3 to switch to stone block. Hotbar tells you which block you're using.
Try this if it lags sometimes:
https://turbowarp.org/556720895
Thanks to Mojang for creating the the original game ("Minecraft").