By one year your child should be able to help you as you dress them by pushing their arms and legs through items of clothing. By 2 years they should be able to remove an unfastened jacket.
By 2 ½ years they can put on easy clothing such as a jacket or open front shirts without zipping/buttoning.
By the age of 3 they should be able to assist with zipping and unzipping and separating the zip at the bottom of a jacket. Between the ages of 3-4 your child should be able to put their hands through both armholes and down the sleeves in front opening clothing (e.g. jacket). They should also be able to take the same item off completely.
By 4 years old children should be able to get their clothes on and off independently but will not be able to manage fastenings (e.g. zips and buttons) for another year or two.
Sometimes you need them to complete the whole process so that they can work out what went wrong. If they put their trousers on without their pants let them dress completely and find the pants at the end.
You can ask questions to help them figure out what went wrong.
You can compare what you do with what they have done.
You can have them look in the mirror to work out what is wrong.
Direct them back to their visual sequence if they are using one.