Draw a long horizontal line AB.
From A draw a line AC perpendicular to AB at A, of length 2 unit. So C is at 2 units from A.
Using compass measure 3 units. With C as the center, draw an arc cutting AB at D. Now AD will be √5 units.
There can be many ways. The above is one way.
Using Pythagoras theorem , we can say that
AD² = CD² - AC² = 3² - 2² = 5
AD = √5 units