Enable cheats in your world
Open the chat and type `/give @p command_block`
Place the command block on the ground
Right-click or tap the command block to open it
Enter a command in the text box
Choose the command block type if needed
Set the redstone mode if needed
Power the command block with a button, lever, redstone, or another activation method
Test the command
Adjust the command or settings if it does not work as expected