For example, if you extend the armhole, you will also need to extend the sleeve cap.
If the change is by a large amount you should use the cut and spread technique, to distribute the extra length evenly throughout the cap (between the notch and the side seam, on both front and back of the sleeve), but if it is only by a small amount (a few centimetres), this method will do the trick.
Take the sleeve you are altering and trace it onto a new piece of pattern paper - leaving space around the outside for changes and seam allowance.
By this stage, you should know what the difference between your sleeve cap and armhole is.
Take the difference and divide by two (you will be distributing it evenly on both ends of the bicep line) and extend the bicep line by this amount at either end, ensuring that the bicep line remains perpendicular to the grain.
Redraw the sleeve cap, passing through the balance notches before gradually steering the line towards the new underarm points).
Redraw the sleeve side seams by joining the underarm points to the wrist at either side.