To have volunteers report back hours completed on their own, please start by creating a new Opportunity - select the "Flexible" template. 
Note: You may want to set the Audience as Private-Open or Private-Closed so only your volunteers can view it. In the example below, we included our Affiliates and Registered volunteers as able to access this Opportunity.
Make sure to choose the "On Your Own" option in the Time portion of opportunity creation.
You also have the option to send reminders to your participants to help remind them to log hours. In the example below, we have chosen to remind participants every two weeks.
Sometimes Opportunities won't take place on a set schedule. Examples of flexible volunteer Opportunities include Board Member Meetings, Membership Meetings, Trainings, Interviews, Specific one-off tasks (like clearing a fallen tree from a bike path), and Tutoring children.