You can't create a memory of something you don't already have in your head. Studying puts stuff into your head for you to remember. Memory usually comes naturally after that happens, but it usually cannot come before, unless you happen to have perfect recall and memorize the image of whatever you are reading.
A good example of committing something to memory on demand is making a mental note. I don't know if everyone has this ability, but it's a pretty commonly known concept. You don't have to study the subject of the note in order to remember it; it's often something simple like "do this tomorrow" that you already understand, so it's easy to memorize.
A good example of committing something to memory on demand is making a mental note. I don't know if everyone has this ability, but it's a pretty commonly known concept. You don't have to study the subject of the note in order to remember it; it's often something simple like "do this tomorrow" that you already understand, so it's easy to memorize.