The permission is not general, and that is worth being careful about. Your surgeon will let you know when it is OK to shower or bathe after surgery. Usually it is fine to shower after 24 hours.
Usually, and after your surgeon says so. Both halves of that sentence are doing work, and an incision that has been left open is exactly the circumstance where the general figure is least safe to assume.
The most important distinction is between showering and soaking. Showers are better than baths because the wound does not soak in the water. Soaking the wound could cause it to reopen or become infected.
If showering is allowed, here’s how. Remove the dressing before bathing unless told otherwise. Some dressings are waterproof, and your surgeon may suggest covering the wound with a plastic bag to keep it dry. If your surgeon gives the OK, gently rinse the wound with water as you bathe, and do not rub or scrub the wound. Do not use lotions, powders, cosmetics, or any other skin care products on the wound. Gently pat dry the area around the wound with a clean towel, let the wound air dry, and apply a new dressing.
For a wound being managed with packing in wet-to-dry dressings, in some cases you can even rinse the wound while showering. In some cases is as far as it goes, and that case is decided by the people managing the wound.
Wound Wizard has a separate page on showering with an open wound generally. This one is about incisions, because the figures and the soaking warning here are specific to surgery.