Hi, that's not easy to describe....first of all you'll have to knit a gauge- most patterns give the knitting gauge at the beginning of the pattern description. Then take some finer wool and matching needles and make a second gauge with these finer materials. Then you have to compare your measurements ( breast, waist, arms, cuffs, etc) with your dolly's and set it in relation to the knitting gauges..difficult to explain- english isn't my native language say your gauge is 20 stitches on 4", waist is, let's say, 25" ( I would dream of that!!)so your pattern says cast on 120 stitches. Dolly's waist is 5" and dolly's knitting gauge is 48 stitches on 4", so you need to cast on for her pattern with the finer materials 60 stitches. When I reduce a pattern I first knit the two gauges and then take my pocket calculator and calculate in % all stitch numbers and measurements. In the case I described avove it means just take 50% of the number of stitches in your pattern and the measurements and it should fit the doll. Of course it is very helpful to fit the work in progress often on the doll . Hope you understood what I tried to explain I have already chosen several vintage " real child" knit & crochet patterns I plan to make for my little Delf Kid darling....hope she will arrive soon!!!!!!
This isn't required, but I find that it's helpful if you first make the item in human size, especially things that require a lot of increasing/decreasing and other shaping, rather than just straight knitting. This gives you a really good idea of how the specific shaping works, and how you can fit it to the doll. It also gives you an idea of what the project should look like at a certain step in the pattern. Plus it gives you a really cute matching doll/human outfit set in the end. ^_~
Even if the pattern gives the gauge, I'd suggest making one of your own, especially if your yarn or needles aren't identical to those used by the patternmakers. In knitting my Potterscarf, I found that I had to add quite a few stitches in order to reach the width suggested by the pattern - my yarn was a different brand, slightly lower quality (but it was all I could get around here, and since it was my first project, I didn't want to order expensive yarn in the event I sctewed up royally - which I did, several times).