1. Check your settings. You need to make sure that you can receive challenges and messages.
Go to https://lichess.org/account/preferences/privacy (or click on your username in the top right of the homepage, click on settings, and then select privacy on the left). Then select the options to enable players to challenge and message you:
2. Starting a game. The player with the White pieces should challenge the player with the Black pieces.
2(a) Look up your opponent on the website
2(b) Click on the magnifying glass at the top of the homepage and type your opponent's username in.
2(c) Click on their name. If they are online a green dot will show.
2(d) This will take you to their page. Click on the crossed swords icon (the one with a TV to the left, not the one at the top of the page).
2(e) Select the correct match options:
Time Control: Real time
Select White (assuming you are the White player)
While we ask the White player to send the challenge, if you're Black and the White player is online and not challenging you then, please send the challenge yourself (just select Black not White).
3a. Once the challenge is issued the player who challenged sees this:
3b. The player who has been challenged sees this at the top right of their homepage:
(And should click on the green arrow to accept. The game will then start.)
4. Playing the game
4a. Pieces are moved by either:
Clicking on the piece, and clicking on the square you want to move it to
Clicking on the piece, and dragging it to the square that you want to move it to.
4b. To resign click on the flag icon twice.
4c. To offer a draw click on the hand icon twice.
4d. You should not normally use the takeback option in a competitive match. However, in completely obvious cases of a mouseslip (e.g. the only logical move is Qa1 captures Qa8, and the Queen instead stops on a7, when this is a ridiculous illogical losing move) then the players can agree a takeback. This is however fully at the discretion of the player to move. Under no circumstances whatsoever should you request a takeback because you've made a mistake/blunder.
5.Talking to your opponent - in an appropriate way, at an appropriate time.
The game has a chat room function. We hope players will treat it like a real game of chess, say hello, exchange briefly pleasantries, play the game without 'talking', and then discuss the game after in a friendly way. Learning from your opponents after a loss is one of the best way to improve, and we hope stronger players will share some insights. It goes without saying (but we'll say it anyway) any abusive messages will result in immediate disqualification from the tournament.
(N.B. In the case of games with junior players, the chat will be disabled and adult players are asked not to communication with the junior player in any way.)
6. Avoiding mouseslips
It is horrible to lose a long game by accidentally making an unintended move. A simple way to avoid it when playing long games is turn on Lichess's confirm move option, which will ask you to confirm you move after you make it.
Click on your name on the top right of the Lichess screen
Click on "preferences"
Click on "Game Behaviour"
Select "Always" next to "Move Confirmation" (see below)
It will hopefully be pretty easy.