Play Tetris Online: A Step-by-Step Guide

Go to an online browser game website such as www.tetrisfriends.com.

Create an account and log in.

Select the game 'Tetris' from the list of available games.

Select the game mode you would like to play: Marathon, 40 Lines, Ultra, or other available options.

Use the left and right arrow keys to move the pieces, and the up arrow key to rotate them.

Make complete lines to clear them, and move quickly to increase your score.

Keep playing until you clear all the lines or reach the highest score.