First copy geometry and then sketch, then copy still life and then sketch, then copy plaster head and then sketch, then copy real head and then sketch. You can practice the human body, landscape, architecture and so on, without any problems.

Basic skills of sketch: arranging cables.

First of all, we know that the point constitutes the first line and the line constitutes the surface. Ribbon cables are used for surfaces. Sketch is not a line, the most basic thing is the ability to restore things. Have you carefully observed the brightness of the apples and pears you drew? Their surface textures are different, so you can't use the same method (line) with a pen. You should imitate their structure, just like our earliest geometric plaster body. Although each one is mainly straight, the arrangement mode and direction are still different.