You can take the measure of  1unit= 2cm for convenience.
draw √2 as shown in the clipping . Then for √3, draw a 90° frpm the point P2,then from the pint p2 join a line of measure 1unit then join the point O and p3 to form a triangle and we get √3. Now for √4 follow the pythogorean theorem i.e. :
  h² = p²+b²
⇒h = √p²+√b²
∴√4= √√3²+1²
In this way from point p3 draw a line of 1unit at the angle of 90° and join it to the point, say P4 and join point P4 with point O, we get √4, then continue the process
till √12 or√13 or any root number you want. I hope my answer helped you.