@foxes here are a few more:
- "your page ought to be same size as the rectangle you made with your fingers when you use them as viewfinder, or the proportions will be very off",
- "drawing an outline of an object first will result in accumulating the errors",
- "if your sketchbook is lying on the table, the image will be stretched and perspective will be all funny because the angle you're looking at it from constantly changes"
Lots of know-hows %)