Click the “+” at the top right and select “Record a new walk…” from the menu. If your current position happens to be close to the end of an existing walk, you will be asked whether you want to start a new walk or extend the existing one. Handy if you want to have a bite to eat halfway through a walk.
You can then start and stop the recording via the red button at the bottom. Every time you stop, the result is saved. On the way, you can also take photos and name them. While you are taking a picture, the recording is stopped for a moment. If the photo is saved or cancelled, recording is resumed.
Once the recording is done, you will still have to edit the walk: a recording often contains too many pointless points and – certainly between tall buildings or trees – these points can also be placed inaccurately. How you do that is detailed here. One hint : start with cleaning the track!