In this research we present a heuristic procedure to obtain, from a practical point of view, the manufacturing planning of a set of products in a manufacturing job shop characterized by multiple products, multiple machines, alternative process routes for the different products and setup times dependent on the machines defined in each manufacturing sequence. Practical constraints related to the availability of machines, raw material stocks, as well as the skills of operators, are incorporated. The existence of set-up times requires the determination of production batches, which turns the problem under study into an integrated lot-sizing and sequencing problem. The iterative resolution procedure combines the determination of production batches, the selection of the appropriate manufacturing sequence and the assignment of tasks to machines while considering the existence of precedence between certain products. An initial lot size calculation mechanism based on the balancing of workloads on machines is proposed, which feeds a genetic algorithm in charge of varying lot sizes and performing the task scheduling. The genetic algorithm incorporates a sequencing heuristic that controls the inventories of previous products and the availability of the necessary skills at any given time.