if you want to make diagrams in a digital format, I'd suggest using something like graphviz rather than drawing
but, if you want to be able to draw as a skill, and be able to use that in both paper/canvas/board and digital media like a tablet
then draw every day, and if it's a diagram, ask someone if what you wanted to convey is understood or not, or if it's a picture (like a portrait or a landscape) draw it several times (can be once a day) until you (and optionally someone else) consider it "good enough" (as in, to improve upon it further, you'd really be gettin on the plateau on the curve of diminishing returns)
I think there was a Show HN where a guy learned to make realistic self-portraits in about a month or so, though maybe the skills needed to do that are not the same as the ones for making diagrams, it might interest you, depending on what you want
but, if you want to be able to draw as a skill, and be able to use that in both paper/canvas/board and digital media like a tablet
then draw every day, and if it's a diagram, ask someone if what you wanted to convey is understood or not, or if it's a picture (like a portrait or a landscape) draw it several times (can be once a day) until you (and optionally someone else) consider it "good enough" (as in, to improve upon it further, you'd really be gettin on the plateau on the curve of diminishing returns)
I think there was a Show HN where a guy learned to make realistic self-portraits in about a month or so, though maybe the skills needed to do that are not the same as the ones for making diagrams, it might interest you, depending on what you want