Things You'll Need
Cones are often made with poster board by rolling the board, taping it in the desired position, and then trimming the opening until it is even. However, this method can prove problematic as the rounded base of a cone can be difficult to trim evenly, especially when made of a tough substance like poster board. These steps can be followed to create a clean cut poster board cone with no trimming needed.
Making A Cone
Arrange the poster board so that one edge is facing you. If you would like a long, thin cone, face the shortest edge towards you. If you would like a short, squat cone, face the longer edge towards you.
Measure where the midpoint is on the edge of the poster board facing you. Make a mark at this point with the pencil. This will be point A.
Tie the string to the pencil. Place the tip of pencil at the opposite end of the poster across from point A, as near to the edge of the board as you can. Pull the string tight and press it down on top of point A with your finger.
Move the pencil to the right. If you keep the string properly taunt, the pencil should curve slightly towards you till it drops off the right edge of the poster board. The point where the pencil line hits the right edge of the board will be point B. Repeat this process going left. The identical point on the left will be point C.
Use the ruler to draw a straight line from point A to point B. Repeat this process going from point A to point C.
Cut along the lines drawn with the pencil. Your board should now look like a triangle with a rounded bottom.
Curl points B and C toward one another till they touch, making the cone's circular opening, and then continue moving the corners past each other, keeping the straight edges on the bottom even with each other until you have an enclosed clone. You should now have a completed cone shape.
Hold the cone together and fix it in this position with the tape.
If you cannot reach across the distance of the poster board in steps 3 and 4, you can fix the string down at point A with tape. If you pull hard the string may come out, but try to keep it as taunt as you can while you draw.
If you have a round circular object, you can use this instead of string to trace the curved bottom of your cone. Just remember that point A must be at the exact center of the circle.