You don't, only your friend can.
I can't play with my friend!

But you were both logged on to the live server at the same time, right? When in doubt you can always try following the instructions:
https://support.chess.com/customer/portal/articles/1444882
Live is in the second half of the article.

But you were both logged on to the live server at the same time, right? When in doubt you can always try following the instructions:
https://support.chess.com/customer/portal/articles/1444882
Live is in the second half of the article.
Too old. And Doing live match still says it
I can't play against my friend because it says "Your challenge does not match the member's current match settings." How do I fix this?