First of all take 2 units in the base the line you have drawn then make a semi circle in the point 2 less than a half then make arc and then draw a perpendicular to point 2. then measure 1 cm with your the compass and make another arc . then from the arc join the point 0 it will be your hypotenuse whose value will be √8 by calculating with the help of pythagoras theorem, which is h square is = p square+b square.
Solution- 8 = 2 square +2 square
:.we can construct √8 as the length of the hypotenuse of a right triangle whose sides are of lengths 2 and 2.
let ox be a number line on which o represents 0 and A represents 2 unit length.draw a line AB perpendicular to O A and mark B on it so that AB =2.
             OB square =O A square+AB square
              =2 square +2 square=4+4=8
Now mark a point P on OX to the right of O such that OP=OB
Then P represents the irrational number √8