1. Open the CS Dashboard.
2. Click on "More..." from the document you want to set, then on "Life Cycle" and "Set to Ready For Check in PLM". This command can be executed on any of the CAD Documents linked to the Item, and as a result, all the CAD Documents attached to the Item will transition to RFC in PLM (this differs from the regular RFC). Please contact the support if this command is not available for you. 

If you want to know more about the criteria, please see the How To below:

What requirement must be met to use Ready For Check in PLM?

Complete documentation: Ready for Check in PLM FAQ