I did it this way.
1. Came to a halt, foot down, nice and steady. One foot still clicked in, pedal at the top of the stroke.
2. Made sure I was steady!
3. Unclick.
4. Lift that leg (quite slowly, to hold balance) so that the underside of my thigh was resting on the saddle. So, standing on one leg with the other cocked over the bike...

5. Slide the saddle under the thigh so that it's next to the knee
6. Slide the leg off the back of the saddle and then over the rear wheel.
All of these movements done slow and easy, so I was always steady.
It was also good if I could stop with my foot-down side next to a wall, so I could lean on it. Nice, but not essential.