make a line AB of the number which has been given to be represented .for eg. if u r given to represent square root of 8 then make a line of 8cm.then extend this line AB and make it of 9cm .now using a compass bisect this line make semicircle from the point of intersection of the bisector using a compass .Now take a protactor and make an angle of 90 degrees on the point from where the line was extended.where the line meets the ssemicricle is equal to under root just take a compass and pass a line through it and intersect it on extended part of ab. the line where meets is under root 8