Step1: go to your options (tab on the top of your screen), scroll down and the last option should be: "spectators can see my hands". You can turn that off.
Step2: Go to the friends tab, type in your opponent's name.
Step3: Tell them to open a table. You go to your matching tab where you can see your friends/followed people's activity. For example "Opponent X: At table [Join]". You click the Join button. You will join as spectator, click "play" in the bottom left to become a player.
Step4: The host goes to advanced options (bottom left again), turns off "players can see spectator chat" and makes it a rated game if you wish to do so. There is also a "player order" button which you can use to make sure each of you starts three games.
Step5: you both click "ready" (bottom left corner again)