How to use CrmMetadataDataSource on form & write to CRM

I'm creating an ASP.NET web form in Visual Studio that will submit data to a custom entity in CRM. I have the basic fields working but I'm not sure how to write option set values back to CRM. I'm using CrmMetadataDataSource to display the option set on the form and that works fine but I don't understand what to do in the code behind file. Can someone help? I've been reading online but nothing is clicking yet.

            <crm:CrmMetadataDataSource ID="optionset" runat="server"
                EntityName="new_entity"
                AttributeName="new_optionset" />
            <asp:DropDownList runat="server"
                DataSourceID="optionset"
                DataTextField="OptionLabel"
                DataValueField="OptionValue" />