The producer starts by determining the production function, which represents the relationship between inputs and outputs. This function is typically represented as Q $=$ f$($L$,$ K$),$ where Q is the quantity of output, L is the quantity of labor input, and K is the quantity of capital input.

Next, the producer identifies the iso$-$quant curves, which represent different combinations of inputs that can produce the same level of output. These curves are derived by holding the quantity of output constant and varying the quantities of labor and capital inputs.

The producer then selects a specific iso$-$quant curve that represents the desired level of output. This choice is based on factors such as cost$,$ efficiency, and production goals.

Once the iso$-$quant curve is chosen, the producer aims to minimize the cost of production while achieving the desired level of output. This is done by finding the optimal combination of labor and capital inputs that lie on the selected iso$-$quant curve.

The producer calculates the marginal rate of technical substitution $($MRTS$),$ which measures the rate at which one input can be substituted for another while keeping output constant. The MRTS is given by the formula MRTS $=-$L$/$K$,$ where L is the change in labor input and K is the change in capital input.

The producer continues adjusting the quantities of labor and capital inputs until the MRTS equals the ratio of input prices. This condition ensures that the producer is using inputs in the most cost$-$effective manner.

Finally, when the MRTS equals the input price ratio, the producer has achieved equilibrium under the iso$-$quant approach. At this point, the producer is producing the desired level of output at the lowest possible cost.

In summary, the producer attains equilibrium under the iso$-$quant approach by selecting an iso$-$quant curve, adjusting the quantities of labor and capital inputs, and achieving a balance between input substitution and cost minimization. This equilibrium allows the producer to efficiently produce the desired level of output.