*This post does not cover what grading is, my post on that is here*
The Craft Yarn Council has an amazing variety of standard sizing charts which I HIGHLY recommend using when grading your patterns. You can find the women's chart here: https://www.craftyarncouncil.com/standards/woman-size
(No I am not sponsored or an affiliate of them, I just use their charts and they're a standard among knitters) There are other sites that give standard sizes as well, this is just my personal preference. It's very important to use these standards, even if you're intentionally going to warp them to make a design for people with longer torsos, shorter arms, etc., because these charts show you what the standard for each size is, so that you have a foundation to start at.
The first thing you need when grading a pattern is your gauge, and your pattern repeat size (if you have a pattern repeat). Take your gauge and narrow it down to stitches and rows per 1 inch or centimetre.
I use excel to do all my calculations, you can use whatever you prefer though. I begin by making a column for each size that I am going to create in my pattern.
Next you'll need to begin adding rows for each number you'll need to calculate, such as neck circumference, body length, arm length, chest circumference, cast on number, etc. I enter the standard sizes in each of my categories first, then make a new row where I calculate the rows or stitches based on my gauge. If numbers aren't whole numbers, I know I need to modify things however much necessary to make my design fit into the sizes I desire.
Note: Sometimes you have to modify your sizes to fit your pattern. This does NOT mean make whatever proportions fit your gauge perfectly. It means that if a size needs to be 33 inches instead of 32 inches in order to fit the pattern repeat, that's okay, AS LONG AS the proportions then remain the same throughout with this modification. It is also important to create a size chart with every size you design so that future customers know exactly what they're getting for each size they will make.
This is just the very basics of grading a pattern, but I hope it helps you get started on your journey. If you'd like me to go more in depth on this topic, I'd be happy to do so :)