To practice your portraiture skills you need models; friends and family are a good option but having someone available is not always possible.
Using photographs is a good way to get away from mirrors and self-portraits for a change; photographs of famous people in particular have the advantage of allowing you to test one of the most challenging aspects of portraiture: the likeness to the subject. Just ask your friends and family who they think the subject on the drawing is!
Below are two portraits I've drawn using photographs as reference. Can you guess who they are? :-)
Both drawings were made with Procreate on the iPad Pro.
The video timelapse of the Donald Trump portrait can be seen here.
The video timelapse of the Hilary Clinton portrait can be seen here.