Toolbar above: Play Icon > 'Create Challenge' window opens > Fill in the different fields for time control, type of game, etc. Choose the tab 'Challenge Friend' and fill in his username > Press 'Send Challenge'. A match request will be issued to that person.
To clear your match offer go to the menu above: View > Console or click the Console icon in the Status area at the bottom left. Type on the command bar under your console: match
For more information, including the command line method, CLICK HERE.