With all due respect, this is one of the most fundamental things in LR and is covered in the help, in books, in tutorials, etc. You just right-click the image in LR, select Edit in, and Select PhotoShop. You can then choose to edit the image with or without any adjustments you have made in LR. Once you're done in PS, save the edited image - "Edit" is appended to the file name and the default format is TIFF. This edited image is automatically added to your LR catalog and you work on it further with LR tools, printing, exporting, etc.
The Lula video course on LR is well worth the cost.