Answer
The carbohydrates attach to the inner membranes of the endoplasmic reticulum and Golgi Apparatus.
Work Step by Step
When plasma membrane proteins get to the Golgi apparatus or the endoplasmic reticulum (ER), carbohydrates can be attached to the proteins. The carbohydrates are inside of the ER and the Golgi so they are attached to the inner membrane side of the vesicles.