Chelsea won 1-0 thanks to Enzo Fernandez's early header to set up a final date with Manchester City.
McFarlane said afterwards: Relief is the correct word. We were confident going into the game. We had a good three days but you never know. We wanted to shift the momentum; our form hasn't been great.
"I did it for a week before (the manager's job), so I kind of knew what I was stepping into. It's easier this time because I have a relationship with the players and I've worked with them for three or four months now. I've been a part of the backroom staff, so I had a feel for what they needed psychologically. It's been intense."
On matchwinner Fernandez, McFarlane said: "The most impressive thing about him is that he's a fighter. You don't have to have the perfect game plan in those moments; he will run and compete and that is often the deciding factor."
He also said of Romeo Lavia's performance: "He's been coming back slowly. I've known Romeo for a long time. I worked with him at Manchester City. I know his level, I know his talent. I thought him and Moises Caicedo were fantastic.
"Whenever they were aggressive, we had an outlet to go into and his quality with bouncing balls and half steps is so important. Liam has done a good job for us and it's good to have two to rely on.