A few things to consider;
1. You can choose an existing or "new" report (invoking the new report wizard). I HIGHLY recommend choosing an existing report. The reason being is that you want this report to be as finished and polished as possible before embedding it. I also recommend that you keep backups of your subreports.
2. The subreport is just another Crystal Object to be inserted onto the Layout. Therefore, take care where you put it. Subreports are almost always used at the Header or Footer level. Putting them into a Details section can greatly increase your refresh time.
3. If there are parameters to be fed within the subreport, these must be linked or the end user will need to fill them out every time.
And this is what you end up with, an "rpt" object. Much like a Notes field, it will grow itself accordingly to it's needs upon refreshing.