How and how often should I apply the balsam for the best effect on rough heels?

The recommended approach is simple but the details matter:

Start with a warm foot soak. Warm, softened skin absorbs rich emollient textures more easily, so 10–15 minutes in warm water before application is the ideal preparation. Pat the feet dry first — applying to dripping-wet skin dilutes the product.
Massage for a few minutes. Work the balsam into the heel area and any other rough spots using circular motions. The massage itself is part of the routine: it helps distribute the product into the textured, thickened surface rather than leaving it sitting on top.

A few practical points from general foot-care experience with rich balms of this type:

Evening use is most convenient. A dense texture needs time to absorb, and applying before bed means you are not walking on freshly coated feet. A pair of clean cotton socks afterwards keeps the product in place overnight and off your bedding.
Consistency matters more than quantity. Calloused skin builds up over months, so a regular routine of repeated application is what gradually changes how the surface feels — a single thick application will not.

Apply only to intact skin and skip any areas that are cracked, broken or irritated until they are in good condition.