Firstly and most importantly, you need to have access to a photocopier that can make copies in different sizes. The next thing to do is have the measurements of the doll that your present patterns fit and the same measurements for the doll you are adjusting the pattern for. You then work out how much smaller (or larger) the new pattern needs to be as a percentage. To do this you divide the measurement of the doll you are adjusting the pattern for by the measurement of the doll of the existing pattern and then multiply by 100. Repeat this for each of your measurements.
The table below shows the measurements of the Existing Pattern on a Cabbage Patch doll and the New Pattern a Little Baby Born doll that I am wanting to adjust the patterns for. I am using centermeters (cm) as it is easier for me to calculate.
As the resultant percentages are between 72.2% and 80.6% I am going to use 75% as an average and the amount that I will be reducing my pattern to i.e. 75% of the original size. Now go to the photo copier, find the reduce button, set that to 75% and take copies of all pattern pieces at this setting. Then make small adjustments where necessary. In my case above, I will add a small amount to the waist as I have reduced the pattern more than I should have on this particular measurement. Now make the outfit up using scraps of fabric and test it on your doll. At this point you can determine if any further adjustments need to be made.
If your doll was larger than the original doll clothes pattern you have, you would simply find the enlarge button and set it to the bigger size e.g. 110% to make the original pattern 10% larger. You will notice that I did not worrying about the length of the arms or legs as these are easy to shorten or lengthen as necessary. If my original doll has an arm length of 14cm and the doll I am making the pattern for has an arm length of 12cm, I would simply adjust the smaller photocopied sleeve pattern so that it was 2cm shorter than the original sleeve pattern length. The same would apply for the leg measurement if you are making long pants.
Although this method does not take into account all measurements and is based on the premise that the proportion of the dolls are the same, it is a great method to use to get the basic pattern for you to start from and is much easier than drafting the pattern from scratch each time you wish to make a new smaller or larger pattern.