Classification, biodiversity and conservation: outline question

A student wants to estimate the population size of a species of grasshopper in a meadow. She uses the mark-release-recapture method. The Lincoln index is: $N = \frac{M \times C}{R}$, where $N$ = estimated population size, $M$ = number caught and marked in the first sample, $C$ = number caught in the second sample, and $R$ = number of marked individuals in the second sample. Outline the procedure the student should follow, explain why random sampling is important in this method, and state one assumption of the Lincoln index and describe how failing to meet this assumption could affect the estimate.