how to draw without reference

To be honest, drawing from reference photos will usually give you a different result than drawing from imagination or memory, which will also give you a different result than drawing from a live model. The real question is whether you can see what an object actually looks like, rather than what your mind tells you that you are seeing.

If you draw from a reference, you have more control over how you set down your lines. You can look at the reference in real time, and place the lines for what you actually see, if you have to discipline to draw what you see versus what you think you see.

If that makes little sense, look at it this way: Drawing an eye from your imagination will probably always look very similar to other eyes you have drawn. It will draw upon whatever experience you have incorporated into your muscle memory, and is less an actual drawing than it is a symbol you have taught yourself to draw.

If you draw an eye from a reference, you will usually spend more time looking at the line and shadow placement, and it will probably look nothing like the eye you can draw from your rote memory.

Reference drawings tend to be more technically correct. The more reference drawings you have under your belt, the better you will understand how objects interact with the space and light around them, and will then be able to incorporate that experience into your imagination based drawings.

However, the best artists in the various fields ALL use references. Disney, for example, is famous for the lengths they will go to provide references for their artists. The fellows over at Pixar likely don't work on a character design without a live model to base off of, first. Even the best artists cannot interpret all the myriad details of what a form will do in light and space without some sort of reminder to refer back to.

My gallery is not very populated right now, but you could look at what I have done and probably tell off the bat that I didn't use any references for the colored drawings I have, because they all look flat and technically incorrect. When I realized that, I went back to the basics and began drawing from photo reference poses that I found on the site, and those look worlds better, technically speaking, than the other paintings, even though they are only quick line sketches.

Like someone mentioned earlier, using references AND your imagination at the same time is really the way to go. That's what the pros do.

TLR, references are a must because they teach you how to see what is in front of you, rather than what you think you should see.