When you've accepted a request or someone has accepted one of your requests, you'll see it in the "Tasks" tab. You can see details of completed tasks as well as ones that are coming up. When it's nearly time to start a task, you'll see a label saying "starting soon."
When you view a task you're going to do for someone that is starting soon, or has a flexible start time, you'll see a "Tap to record start" button and a timer with the requested amount of time. Tap the button to start the timer when you're ready to start the task. You can close the app once the timer is running.
The button will change to let you record the end of the task. When you record the end, the app will automatically calculate the number of points that should be transferred. If you need to change the time that was recorded (for example because you forgot to stop the timer), just ask a group administrator – they can adjust the time and points as needed.