Take your actual body measurements as they are more accurate than measuring over your clothes.

Collar
Measure around the base of the neck where the collar sits.

Chest
Measure the chest at the fullest part, placing the tape close up under the arms.

Waist
Measure the natural waistline.

Inside Leg
Measure from the crotch to where your trouser is normally worn on the shoe

Jacket Length
Measure from the centre back of your neck to your hemline. Then length measurement will vary depending on the style of garment.