You can't make clothes for your cat if you don't know some standard measurements, nor where to start. If you have some experience with dog clothing, then you are already a step ahead, taking cat measures is very similar to taking dog measures. If you dont know anything, nor what size is your cat, you can start over here.
You will need a measuring tape, paper, and a pencil to take notes.
Now, measure cats is not easy thing, unless your cat is kind of lazy and just wants to be laying on any surface all day, and even like that is very difficult; to take measures, your cat needs to stand up in his/her four paws, or you won't get accurate measurements, my personal advice, is to wait a moment when the cat is calm, or when its looking something on the wall, its tricky, but i'm sure you'll figure out how, every cat is different.
Take your cat's measurements:
TORSO:
Is the size of your cat torso, at the widest point, generally, the widest point is right behind de front legs, but is better if you use the thickest part of the body, especially if you have a little fur ball (yes, I mean fat cats).
Wrap the measuring tape around the cat's rib cage (or the thickest part as we said) and on paper, write down the number you got as "Torso".
NECK:
This measurement is taken around the cats neck, take a look at your cat, it's usually where their collar would naturally sit (neither too tight nor too loose). Wrap the measuring tape around the neck, and measure the circumference. now, on paper write down the number you got as "Neck".
BACK:
You take this measurement from the base of the neck (where you took the neck measurement) to the base/start/beginning of the tail (not the tip, just no, not the tip). On the paper, you write down the number you got as "Back".
CHEST:
This one is optional, in fact, this is especially if you have a little fur ball (yes, fat cats).
You take this measurement from the base of the neck (at the front) to the beginning /at the front of the legs.
And there you have it, now you have all the basic measurements of your cat.