For smooth(er) mashed potatoes, make sure your potatoes are tender after boiling and use a potato ricer (which breaks them down better than a regular masher). I linked to the ricer I have in my recipe card. Note that this still isn’t guaranteed to give you perfectly smooth mashed potatoes; you’d need to run them-through a fine mesh sieve for that, which is a lot of trouble and can result in gluey potatoes.