My experience is that it is not possible enter such a flow state on demand. The state is beyond "my" control. There appears to be "a part" (not necessarily a physical part) of us that we think is "me" that wants to control, i.e. micromanage, and "on demand" means that this part would decide, i.e. able to control the flow state. However, the state seems to happen when this part lets go of the need to control things, and seems to disappear when this part tries to control things. The whole discussion is too long for here, but I recommend book Inner Game of Tennis for details.