Take your ruler and a pencil and construct a segment of any length on a piece of paper as shown below

Take your compass. Make your sure that the pencil is included in it.

You may have to put one hand where the needle is and one hand where the pencil is.

Then open or close the compass making sure that the distance of the opening is bigger or smaller than the length of segment AB

For this situation, we will open the compass to a distance greater than segment AB

Put the needle of your compass at A and draw an arc

Draw the segments from the two endpoints to the point where the two arcs intersets