Catrin, when I first started with clipless pedals, I used the dual-sided SPDs that you're planning to use, and I did more or less what you have in mind. I sometimes rode clipped in, others times I didn't or with only one foot clipped in. Before I truly got used to using them religiously, however, I bought my road bike and had Look Keos pedals installed immediately. That's when I truly learned to use clipless pedals--primarily because I had no other choice at that point.

From there, I mostly did what Muirenn did. I went to a parking lot and practiced stopping over and over and over again.
In my own personal experience, it's all well and good to ride down a straight road clipped in, but you're likely only going to get proficient at using clipless pedals if you practice coming to a stop with them. The act of unclipping, slowing down, leaning a bit to unclipped side and putting your unclipped foot down has to become largely automatic, And it only becomes automatic by doing it repeatedly. You also have to practice getting moving and clipped in again with them, which can be almost as hard.
The best place to do this, IMO, is on a road/parking lot/route where you're going to have few other distractions. Always unclip with the same foot and otherwise follow the same routine each and every time. In time, it should become automatic.