The main symptom I found was rough idle and surging while the car was stopped. The engine would move up to 1200 RPM and then drop down to under 500 RPM before recovering. Having the AC on or off would not change this. The car had 40, 000 miles at the time and I would bet the throttle body had not ever been cleaned before. So expect to clean the throttle body about every 30,000 miles so it doesn't get to the point where the engine struggles.
Start by opening the box holding the air filter. There are three clips that keep the top latched. Pull them away from the lid to loosen them. It can take some pressure to get them to move. Get ready, it'll take even more pressure to put them back on at the end of the job. Some earlier Focus models had a life-time air filter in a sealed box. I don't have any experience with these filters, but the removal process for these models will be different.
Once the lid to the cold air box is undone, remove the air filter and set it aside to protect it from damage. The hose attached to the lid goes to the throttle body. One hose clamp holds it on and it can be removed by twisting the screw on the clamp.
Move the hose and air box lid as far out of the way as you can. You're limited on movement because of sensors still attached to the air intake, but it doesn't have to move far for you to get to the area you'll need to clean.
This is what the inside of the throttle body looks like. I was surprised how clean it looked for as bad as the car was idling. It turned out all the gunk was on the other side of the bronze colored throttle plate.
Find a soft plastic shim to hold the throttle body butterfly plate open while you clean the inside. I used the handle of a small screwdriver. Use something big enough to hold the plate wide open and so it won't fall in, but small enough you can work around it.
Use a can of throttle body cleaner to wet the walls and plate of the throttle body. Let it soak for a few seconds and start cleaning everything with a rag. Make sure to clean the thin edge of the butterfly and as much of the walls as you can. Really focus on the edge of the plate and the wall area it butts up against. That's the gunk causing the issues. Respray and wipe away the dirt until you've got it as clean as you can. Close it all up and start the car. It might not start or run like normal at first because of the liquid cleaner still in the intake and the computer trying to control the newly cleaned throttle body, but after a few moments the idle will even out and should remain even from now on.