It is a skill that we take for granted but one that doesn't come natural for our children with disabilities. With one of our goals being independence one day, my wife and I have been working with John on multiple skills and learning how to dress himself is one of those skills.
We began about 6 months ago with teaching him how to put his shirt on by putting his arms through the shirt first and then slipping it over his head. Once he got used to this, we moved into taking his shirt off. This was a little more complicated because it requires more steps. And that is what we did: break it down into steps for him. We still have to remind him of hand placement in order to pull his arm out of the sleeve, but other than that he is making positive steps in being able to dress himself.
Part this skill is learning to put away the clothes you take off. In this case, he was putting his pajamas back in his bed so he could wear them again tonight. He doesn't do this automatically, but then again neither do my other children. I tell him to pick his clothes up off the floor and take them and put them in his bed. He sometimes obeys and sometimes he doesn't. When he doesn't, we just take him by the hand and have him pick up his clothes and we then lead him to where we want him to put them up.
As for the shorts/pants, he can get them on. He sits down on the floor and slides them on, pulls them up past his knees and them stands up to pull them up. It is much easier for him to pull up the front than it is the back. But this is a work in progress.
I often catch myself starting to do this for him, mainly because I 'think' that my life should be so rushed that I don't have time to wait for him. But then I think of how we need and want to train him to live independent one of these days, and that doing things for him is not treating him fair.