How can I use leftover fat?

It depends on what kind of leftover fat. Like when you roast meat/chicken and the residue/fat that left on the pan. When panfried bacon, salmon bellies, or pork fat. Its best to drain it away or paper towelled it as much as you can than try to use it to clog your arteries later on in life. Sometimes,if you like, you can use the leftover fat to flavor gravy or sauces for the dish. Pork fat or lard to stir fry veggies or use it as fat like butter/shortening for some asian pastries and cakes. *

I use leftover bacon fat in biscuits. You didn't say what kind of meat this fat is from and that might determine the best use for it, but you can use animal fat for frying.

Gravy

Cooking eggs, veggies etc..

Make biscuits... use about 2/3 of the shoertening called for, and use the rest of well chilled and solidified fat. YUM

In olden days, when cooks used to be more careful about throwing anything away, a common sight on the kitchen counter next to the stove was a coffee can to hold bacon grease. Frugal (and smart!) cooks would then use the grease in various ways in their cooking.

You can use it for frying but it isn't really healthy and tends to smoke and taint the taste of the foods you cook

Dune

Lard for cooking. You can fry eggs, cook green beans with it etc.

use it To get atherosclerosis and obesity.

Yeah use it for that