Not everybody does the unclip one foot and lean thing. I don't and I am aware of many other cyclists that don't. So Catrin, do what works best for you. Take the advice in this forum and from your trainer, and figure out what works best for you.

I unclip both feet in advance of the anticipated stop. I have two-bolt SPD pedals, so I wear high end mountain bike shoes or cycling sandals, which means I can pedal on the back end of my shoes and not have pedals that flip over. If I don't have to stop after all, I clip both feet back in quickly and off I go. If I want to approach the intersection slowly, I can pedal very slow or teeter, then clip back in as I cross the intersection. Or I can come to a halt, plant both feet on the ground, and there is no falling over at all. I clip my right foot back in while I am waiting to cross the intersection, then off I go when it is feasible. I unclip one foot and lean when it needs to be an emergency stop.

All I am saying is just because cyclists tell you that you have to unclip one foot and lean, it doesn't mean you have to do it that way. Figure out what works for you, and do it that way.