I am one size at the bust and two sizes bigger at the hip, do I cut the bigger one and take it in, or is there a proper method?

Before you grade anything, do the step that removes most of the guesswork: measure the flat pattern, not just yourself.

Size charts describe bodies. The pattern piece describes the garment, and the difference between them is the ease the designer intended. Two patterns with identical size charts can be drafted with completely different amounts of room, which is why the same nominal size fits differently between brands.

So lay the pattern out and measure across the bust, waist and hip lines, minus the seam allowances, and double it for the full circumference. Compare that against your own measurements. The difference is the ease you will actually get.

What this tells you:

Whether you need the size you think you need. Sometimes the pattern already includes enough room at the hip that you do not need to grade at all.

Whether the garment is meant to be fitted or loose there. A pattern with a lot of ease at the hip is designed to skim, and grading up two sizes gives you a garment two sizes too big in a way the chart did not warn you about.

Many patterns print finished garment measurements somewhere on the envelope or the tissue, which saves you the measuring. It is the most useful number on the packet and the least read.

It is what you should have been doing, and it is far less advanced than the word suggests. Grading between sizes is drawing a line from one size to another. That is genuinely it.

Why your current approach fails is worth stating, because it explains the frustration: cutting the largest size makes the shoulders and armholes bigger too, and those are the two things a tailor cannot easily fix and the two things that decide whether a garment looks right. You have been sizing for the easiest part of your body to alter and compromising the hardest.

The method:

Cut the size that matches your bust and shoulders, because that is where the unfixable structure is.

On the pattern, mark the size you need at the waist and at the hip on the relevant lines.

Draw a smooth curve between the marks, moving gradually rather than stepping out. Blend over a good distance - from under the arm down through the waist to the hip, so there is no corner.

Do the same on front and back, and check the side seams still match in length.

That is the whole technique. A pencil, a ruler and a curve, and it takes ten minutes.

The result fits at the shoulders, which is what everyone notices - and has room where you need it.

Two practical notes for when you start.

Trace, do not cut the original. Trace onto paper and grade on the copy. You will want to try the same pattern again with adjustments, and once you have cut the tissue that option is gone. This is also what makes grading low-risk, you can be wrong and start again.

Check the seam lengths after grading. The commonest error is a front and back side seam that no longer match, because the curve was drawn differently on each. Measure both from armhole to hem and adjust before cutting fabric.

Grade the facings and waistbands too. Everybody forgets these, then discovers the waistband is two sizes smaller than the skirt it has to go on.

And one thing that will save you a lot of time once you have done it twice: write your grading down. Bust size, waist size, hip size, and where you blended. It will be nearly the same on every pattern, because it is a fact about you rather than about the pattern. After two or three you stop working it out and start applying it.

The shoulders-first principle in the first answer is the thing to carry into every pattern from now on. Almost every fitting problem people describe as being between sizes is actually a garment sized for the wrong part of the body.

One thing nobody has said: make the toile in something that behaves like the final fabric. I graded beautifully, tested in cheap cotton, and the drape in a lighter fabric moved the whole fit enough that the careful grading was irrelevant.

It does not have to be the same cloth. It has to have roughly the same weight and stretch.

Two sizes between bust and hip is completely ordinary, for what it is worth. The charts are the odd thing, not you.