Steps of root nodules :
(1) Rhizobium bacteria makes contact with a susceptible root hair and divide near it. 

(2) After successful infection of the root hair causes the root hair curls,

(3) Infected thread carries the bacteria to the inner cortex. The bacteria  modifies into rod-shaped bacteroids and cause inner cortical and pericycle cells to divide. Division and growth of cortical and pericycle cells lead to the nodule formation,

(4) A mature nodule forms with vascular tissues continuous with those of the root.