how to make custom creation method [Django models]

Using Django
1 
class Boat(models.Model):
    name = models.CharField(max_length=100)
    quantity = models.DecimalField(max_digits=8, decimal_places=2)
    fish_type = models.CharField(max_length=50, choices=FISH_TYPE)

class Load(models.Model):
    boat = models.ForeignKey(Boat, on_delete=models.CASCADE)
    delivery_date = models.DateTimeField(auto_now_add=True)

class OrderLoad(models.Model):
    order = models.ForeignKey(Order, on_delete=models.CASCADE)
    quantity = models.DecimalField(max_digits=8, decimal_places=2)
    fish_type = models.CharField(max_length=50, choices=FISH_TYPE)

what I want to do is, when orderload instance created, it should reduce the quantity.

2

You can add that logic to the save method for OrderLoad.
There are some possible “gotcha’s” depending upon your application. Whether or not you’re going to be affected by them depends a lot on the specifics of all the different ways that OrderLoad instances can be created or updated.