Enable cheats in your world
Open chat and type `/give @p command_block`
Place the command block on the ground
Right-click or tap the command block to open its interface
Enter a command in the text field
Choose the command block type if needed
Set the redstone mode if needed
Power the command block with redstone or set it to always active
Test the command
Edit the command if it does not work
Chain command blocks after a previous command block if needed
Use repeating command blocks for continuous commands
Save and reload the world if necessary