Press ruler on the dotted line, then use the knife back keep near to the ruler and make a light crease along the dotted line. Then it folds naturally & easily into 90°, as shown at the right picture.  Avoid making strong creases, as this will cause the paper to tear.

Many modelers, particularly European ones, prefer glues made by a company called UHU. This transparent glue is good & durable. Avoid over application as this may cause the paper to wrinkle.

Mainly used for handle or fold small parts. The appearance of the small parts looks more beautiful by using this auxiliary tool.

There are many curved parts should be made.  The curved surface looks more beautiful by using round pencil or round stick.  Put the pencil or stick on the paper & roll over, then the paper curls smoothly.