How are lysosomes formed?

Lysosome formation is a multi-step process that involves the fusion of vesicles from the Golgi complex with endosomes. 

Endosomes are vesicles formed when a section of the plasma membrane invaginates, engulfs extracellular material that enters the cell, and then pinches off from the membrane. This membrane-bound structure is called an endosome. Late or mature endosomes fuse with other cellular structures including transport vesicles from the Golgi complex, which contain enzymes such as acid hydrolases.  On fusing with transport vesicles, the endosome undergoes chemical modifications and eventually develops into a lysosome.