Without further ado, here are instructions for a full bust adjustment. Read these if you're doing a small bust adjustment, since the alterations are the same, just reversed (more on that at the end of the post).
1. Start with the D cup pattern piece for your size. (I'm using the C cup in the photos below, so just ignore that). Mark in the lines shown in the picture below.
2. Slash along the lines, leaving a hinge at the top of the pattern piece. Open the slashed lines the amount you need to add to the bust. I added 1", which should gain you 1-2 cup sizes. Tape the pieces to a fresh sheet of paper.
3. Redraw the center front line.
4. Re-draw the dart legs, from their original leg points, to a new end point in your slash. Your dart will be much bigger now. When this pattern is sewn, the dart excess gets cut away, so you don't need to worry about all that excess fabric. (Another option is that you could gather the lower edge of the cup piece rather than having a dart. This is pretty common in slips.)
5. True the dart legs: close the dart as it will be sewn. Even up the bottom edge of the pattern piece if needed. Roll over the lower edge of the dart with a spiky tracing wheel.
Open up the dart and draw over the perforated marks left by the tracing wheel. This is your new cutting line.
6. That's it! Test out your new cup piece in a cheap 2-way knit fabric. To get the correct fit, it's very important that your fabric for the test and the final garment stretch in the lengthwise and crosswise directions.
For small bust adjustments: you can use these same steps, but instead of spreading the pattern piece apart, overlap the pieces, making it smaller. Your piece will get narrower and shorter, and your darts will be smaller.
Let me know if you have any questions. Next, we'll be cutting and marking our fabric!
Previous sew-along posts:
Slip Sew-Along #1: Inside the Kit, Plus Other Supplies
Slip Sew-Along #2: Pre-treating Your Fabric
Slip Sew-Along #3: Picking a Size and Making Adjustments