Here is a list with the correct ways to measure as well as sample numbers from an average/small sized female body so you know if you're getting the right kind of number. Of course these numbers are not to be 'ideal' they are just an example so you know if you're measuring right.
Neck: Measure all the way around the neck. ex: 13
Shoulder: Measure either from the front of the shoulder socket to the front of the shoulder socket, or from the the middle of the arm to the middle of the arm. ex: 15/17
Bust/Chest: Measure all the way around the chest, centering around the nipples which should be the farthest out part of the chest. ex: 35
Under Bust: Measure all the way around right under your bust/bra. This is important for certain figure types and when making tight clothing. ex: 29 1/2. (The number is sometimes larger than the bust size depending on figure)
Waist: Basically where the bellybutton is. Measure all the way around the area that goes in the most on your stomach area. ex: 30
Hip: Measure all the way around where you feel your hips protruding the most. ex: 35
Buttocks: Measure all the way around where your buttocks sticks out the most. This is important for females more than males, and for tight pants. If this number is more than 5 inches more than your hip measurement you may have to consider modifying the back of pants and skirts to be a bit longer so it fits snugly.
Sleeve Length: Measure from the shoulder (or seam on your shirt where the sleeve starts) to your wrist. ex: 22
Pant Length: Measure from your hips (or where you like to wear your pants) to your ankle/end of pants. You will want a friend to help you because measurements can get messed up if you bend over to measure yourself. ex: 33
Pant Inseam: This is your pant leg length starting from the crotch of your pants. Measure from where you like to have the crotch of your pants to your ankle/ end of pants. Male pants always have more space for the crotch. ex: 28
Wrist Width: Measure all the way around the wrist. This is helpful for sleeves and gloves. ex: 5 1/2
Forearm Width: Measure all the way around the largest part of the forearm. This is the area below the elbow. This measurement is vital when making anime gloves and those arm warmer thingies anime characters are always wearing. ex: 8 1/2
Forearm Length: Measure from the elbow to the wrist. This is also useful for anime gloves or arm warmers. ex: 8 1/2
Shirt Length: Measure from the top of the shoulder (or seam on your shirt on your shoulder) to your hips (or where you like your shirt to end). Keep in mind that your bust may alter the length. The front and back may be different, so make the sides even. Choose the longer measurement (the front).
I hope this guide helps! The rule of thumb is... measure twice cut once! Also I always cut more than I need in case I made/make a mistake.