For normal use, a new time is added by the start and stop function on the dashboard.
This help is for when you forgot to add an earlier time.
Go to the history-view and long-press on the day and hour where you want to insert the time. A create time dialog should now be shown. Edit the start and stop time to match the work length and then choose the project to add the time to.
If the time overlaps another time, that time will be clamped to not overlap anymore.