First select sub report from Toolbox and put where you want to show.See the image bellow

Now right click on sub report properties and type your sub report name .

Now You have to create a sub report even handler in your .cs file from where you load your report like that:
public Ctor()
{
string path = HttpContext.Current.Server.MapPath("Your Report path");
ReportViewer1.Reset(); //important
ReportViewer1.ProcessingMode = Microsoft.Reporting.WebForms.ProcessingMode.Local;
ReportViewer1.LocalReport.SubreportProcessing += Process_Subreport;
LocalReport objReport = ReportViewer1.LocalReport;
objReport.ReportPath = path;
// Add Parameter If you need
List<ReportParameter> parameters = new List<ReportParameter>();
parameters.Add(new ReportParameter("Name", Value));
ReportViewer1.LocalReport.SetParameters(parameters);
ReportViewer1.ShowParameterPrompts = false;
ReportViewer1.ShowPromptAreaButton = false;
ReportViewer1.LocalReport.Refresh();
//Add Datasourdce
ReportDataSource reportDataSource = new ReportDataSource();
reportDataSource.Name = "Datasource Name Used due to report design";
reportDataSource.Value = DataSourceValue;
objReport.DataSources.Add(reportDataSource);
objReport.Refresh();
}
Now Create Even Handler Method to load sub report details.
private void Process_Subreport(object sender, SubreportProcessingEventArgs e)
{
//You can get parameter from main report
int paramname = int.Parse(e.Parameters[0].Values[0].ToString());
//You can also add parameter in sub report if you need like main report
//Now add sub report data source
e.DataSources.Add(new ReportDataSource("DataSource Name",DataSourceValue)));
}
I think it will be works for you.Thank you.