I sometimes struggle with this. Sometimes I can just click my fingers and voila, whatever I want appears, but sometimes it just will not happen. Like others have said, you can't really have any uncertainty in my mind. You have to know that you can do it.

A good technique for me is to imagine something behind a door. Open the door and it'll be there (and have no doubt about it). I like some of the ideas already mentioned e.g. imagining things in drawers, beneath blankets. You should have a go at some of these techniques.