Chest: wrap the tape around the fullest part of your chest, under the arms, keeping it level across your back. Breathe normally — don't inflate your chest — and keep one finger of slack under the tape.
Waist: measure around your natural waistline, roughly at navel height, not where your trousers currently sit. This is the number that determines how a blazer-kurta will close at the front.
Shoulder width: measure straight across your back from the edge of one shoulder bone to the other. This one is easiest with a second person's help, since it's hard to keep the tape level on yourself.
Length: for tops, measure from the base of the neck (where a collar would sit) straight down to wherever you want the hem to fall.
Every Fabric Kit purchase includes the option to add custom in-house tailoring at checkout, where these four measurements feed directly into our tailoring queue alongside your fabric and style selection.