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.

Put the needle of the compass at endpoint A and adjust your compass so that the tip of your pencil touches endpoint B

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

Constructing an isosceles triangle