Lots of good advice and I'll just reemphasize that it is going to take awhile. This subject has been posted before and the key word if you really want to use flats is commitment. Don't be that douche on Youtube that uses them three times and then makes a video about why they suck. I bought my first SPDs in 1994 and always used some type of clipless pedal until about two years ago. I ride with a couple guys that make riding flats look easy, had just built up a new bike, so I figured I would give it a try. It was an interesting, sometimes very frustrating, learning curve and lots of bad habits to overcome. It took me the better part of a year for it to feel natural.