In order for the paper model to have smooth and regular fold lines, just before the fold, you need to use some blunt object (this can be a mechanical pencil, a non-writing ballpoint pen, etc.) and draw it along the fold line, making a small groove.
Usually, this procedure is performed before cutting out the parts of the paper model: it is carried out with a blunt tool along the fold lines, making a small groove. Thus, when you try to bend along the bend line, the part will bend exactly where you drew the tool.
To get very sharp edges when making models from thick paper (cardboard), you can draw along the fold lines with a paper knife, but not completely, but cutting only the top layer of the paper. With subsequent bending, its edges will be sharp.
