Step 1: Define diazotisation.
Diazotisation is the reaction of a primary aromatic amine with nitrous acid ($HNO_2$) under cold, acidic conditions to produce a diazonium salt ($ArN_2^+X^-$).
Step 2: Generate nitrous acid in situ.
Nitrous acid is unstable and is generated in situ by adding sodium nitrite ($NaNO_2$) to a cold, dilute solution of hydrochloric acid: $NaNO_2 + HCl \rightarrow HNO_2 + NaCl$.
Step 3: Temperature control is critical.
The reaction must be maintained at 0 to 5$^\circ$C (273 to 278 K). At higher temperatures, the diazonium salt decomposes rapidly, liberating nitrogen gas and forming phenol or other by-products. Ice is used to keep the temperature low.
Step 4: Write the equation for aniline.
$C_6H_5NH_2 + NaNO_2 + 2HCl \xrightarrow{273\text{-}278\text{ K}} C_6H_5N_2^+Cl^- + NaCl + 2H_2O$. The benzenediazonium chloride formed is an important intermediate for coupling reactions and other transformations.