Step 1: recall the goal of the Etard reaction.
The Etard reaction is a neat way to stop the oxidation of toluene exactly at the aldehyde stage, so the methyl group of toluene becomes a $\mathrm{-CHO}$ and we get benzaldehyde rather than going all the way to benzoic acid.
Step 2: choose the reagent and conditions.
Toluene is treated with chromyl chloride $\mathrm{CrO_2Cl_2}$, usually in a solvent like carbon disulphide $\mathrm{CS_2}$. The chromyl chloride attacks the methyl group and locks itself onto it, forming a stable brown chromium addition complex that cannot be over oxidised.
Step 3: free the aldehyde.
This brown complex is then stirred with water (acidic hydrolysis). The water breaks the complex apart and releases benzaldehyde.
\[ \mathrm{C_6H_5CH_3 \xrightarrow{CrO_2Cl_2,\ CS_2} (complex) \xrightarrow{H_3O^+} C_6H_5CHO} \]
Final answer: toluene reacts with chromyl chloride in $\mathrm{CS_2}$, and the brown complex is hydrolysed to give benzaldehyde.