However, if the subreport has parameters, we must decide what to do with them. Because if you leave them unlinked, the end user is prompted for them alongside the parameters for the "main" report.
The idea is to select a parameter from the "Available Fields" list, add it to the right side and then select the subreport parameter it should be linked to. This way, the end user (in this case) is only prompted for one date range instead of two.
A few considerations;
1. The data types in the main and subreport(s) must be identical when Linking, otherwise they won't show in the drop down. This literally just happened to me a moment ago when producing this screenshot.
2. Do yourself a favor and name your parameters consistently. In this example, the Date_Range parameter from the main is linked to the Date_Range parameter in the sub.
3. If there is no corresponding parameter in your subreport, you can try using the "Select data in subreport based on..." checkbox. Results vary. If you're willing to embed a subreport, go the entire distance by creating matching parameters.
And as ALWAYS keep backups.