Soaking potatoes in water for around 5 to 10 minutes before cooking them does help to remove some of the starch especially from the surface of the potato.
There still may be some starch leftover in the potato even after soaking the potato but when you do soak the potatoes you do get rid of at least some of the starch.
All you need to do is soak the potatoes in a bowl of cold or lukewarm water for 5 to 10 minutes or a bit longer.
Then dump the water out in the drain and cook your potatoes as normal.
If you need to avoid starchy foods but want to eat some potatoes you can eat the potatoes after soaking them which is what my moms doctor recommended she do.