Use Russet Potatoes for ideal starchiness
Start with cold water
Add salt to the cooking water
Cut potatoes in half lengthwise, don't cut into cubes, this will help the potatoes cook more evenly, you don't want starchy water. If you cube, the smaller cubes will cook faster and break down, giving you very starchy water, and not nearly as good of mashed potatoes.
Simmer, don't boil potatoes, so they cook evenly.
Cook for 20-25 minutes, error on the side of overcooking not under-cooking, you want fork tender. If you undercook you will never get rid of the lumps.
Drain fully then return to pot, and turn heat up to evaporate excess water. This is necessary, otherwise your consistency will not be right.
Mash with a wire masher. Do not over mash, as this makes them glue-y. You want to break potatoes up before adding seasonings and fat. So, once you have mashed, add butter. Then mash again. Then add milk and sour cream.
Finish the potatoes using a wire whisk to add salt and pepper. This will result in light and fluffy, not glue-y, not over starched, mashed potatoes.