Eh, not really. I just kind of do it. I don’t know how. I’m sure it could be learned, I assume all it is is learning how to control a particular muscle. I’m not even sure what I’m doing when I do what I do. I might be dilating my pupils, or I might be doing something else. All I know is that when I do it my vision blurs, and lights and such look bigger, as if my contact lenses aren’t in.
The following is highly theoretical:
This might help, but I’m sure it can’t be good for your vision. If you don’t wear glasses, find someone who does and put on that persons glasses. (Or f you wear contacts, find someone with a significantly higher prescription and put that person’s glasses on while you have your contacts in. Or if you wear glasses, try putting the other person’s glasses over yours.)
You should find that although everything is blurry initially your eye will be able to adjust, at least for a few seconds, making everything clear. If you can figure out what your eye is doing as it adjusts you might be able to learn how to adjust it voluntarily.