Start off by peeling your potatoes using a knife or a potato peeler. Then give them a good rinse.
Then cut the potatoes into 1-2 inch cubes. Make sure the potato pieces are similar in size for even cooking.
Place the cut potatoes in a large stockpot and fill it with enough water to submerge all the potatoes. Roughly about 5 cups of water. Make sure the water is cold. That way the potatoes cook as the water heats up. 
Now add 1 1/2 tablespoons of sea salt to the potatoes and bring the water to a boil.
After the potatoes have boiled let them cook over medium-low heat for about 15 minutes. Or until the potatoes are fork-tender.
Carefully strain the water out of the pot while the lid is cracked open to keep the potatoes from falling out.
Now add the softened butter, sour cream, heavy cream, and pressed garlic.
 Using an electric hand mixer, beat the mashed potatoes until smooth and creamy. Season as needed with salt. 
Notes
Use cold water - By cooking the potatoes in cold water, the water heats around the potatoes, so they cook evenly, all at the same time.
Cut to similar sizes- Cooking the same-sized potato pieces is important to even cooking. If some are smaller, they will cook too fast, and larger ones too slow.
Use warm cream and softened butter - Softened butter and warm cream will help the potatoes mash smoothly and be creamy. Too cold ingredients can lead to chunkiness, as can pieces of cold butter.
Add garlic to enhance the flavor - Garlic goes really well with butter and potatoes, enhancing their natural flavors and adding a delicious, pungent note to the dish.