The lumen of the golgi apparatus has enzymes that will covalently bond carbohydrates to proteins.
In the diagram below an integral membrane protein is indicated by a red line
The green hexagons represent carbohydrates.
the carbohydrates are covalently bonded to the part of the integral membrane protein that is in the lumen of the golgi (1)
the glycosylated protein is budded from the Golgi (2-4)
the vesicle that has the integral membrane protein, fuses with the plasma membrane by the process of exocytosis (5-8)
the glycosylated part of the integral membrane protein is now on the outside of the cell (9)