If this was a straightedge and compass problem then I would agree with PenasRaul that it's not possible to make an ellipse. However we are given a few more things to work with. 1) a ruler and 2) a pair of scissors either one can be used to produce an ellipse. I say ellipse because trying to make a perfect ellipse in real life is like trying to make a perfect rectangle in real life (the existence of which could break both euclidean and non euclidean geometry). If you want a perfect ellipse use a computer. Otherwise, please continue to read.
As there is already an answer to how to use a ruler to make one, so I shall go through a step by step process with the use of the scissors. There are a few way to do this as well but I like this one the best. (Oh And I will only use the ruler as a straightedge btw.)
1) Use the ruler and scissors to cut a strip of paper along the longest edge of the paper. This doesn't have to be exact, but it should be about as wide as the diameter of the pen or pencil.
2) (optional) roll the strip into a cylinder. This will make things easier to use, but it is not as necessary for what we're going to use it for. you can also fold it in half or fourths (if you don't want any frayed edges).
3) At one end make a loop and thread the other end through the loop and looping it back through to make a square knot. Secure the knot, doesn't have to be tight.
4) Set both compasses to be at right angles. This can be done by constructing a perpendicular bisector with the straightedge and a compass. Once that is done, hold onto both of the compasses in one hand such that the needles are jutting out, making sure that the paper loop can fit over it. This will act as foci.
5) Place the paper loop made in step 3) around the two compasses and place the needles on the paper.
6) Use either the pen or the pencil to trace the ellipse using the paper loop as a tracing guide.
While the paper loop is a makeshift string, it was constructed using the materials at hand. So No magical can opener.