down tilt is really valuable as a poke and forcing people to shield or respect when you'll dash in (they'll try to dd grab/whatever counter move you have, so you can dash in, dash away and then grab whatever they whiff if they expect your dtilt). the best ways to dtilt are wavedash dtilt and run cancel dtilt (dash then hold down, press a asap). you should also learn to cancel your dtilt asap by dashing away or doing another move after it. don't jump because it leaves you vunerable unless you use it to entice someone, but try to aerial as little as possible unless your opponent has jumped or you're zoning/protecting yourself with fair when your opponent runs in to attack you. dash dance grab is really strong, if you've conditioned your opponent to shield when you dash up it's strong too. then depending on the matchup, you can either upthrow or fthrow/dthrow them and try to get them off the stage or on to a low platform so you can juggle/edgeguard them off the stage for a kill. get used to having to bair/dtilt a lot to kill your opponents, you don't want to have to rely on fsmash for kills because it's pretty difficult to set up and marth excels when his opponent is in the air above him (depending on the MU) or cornered/off the edge and pressuring them with movement and the occasional dtilt. also dd sh fair is pretty good if they rush at you with an aerial, sometimes it's easier to get and covers more space, just be careful of their cc/oos options. try to get your opponent out of cc% asap with throws and juggling too since marth gets wrecked by cc if you make mistakes. you really have to optimise your punish game and end their stock in as few grabs as possible (regrabs not counting, I mean grabs/hits from neutral). don't approach with aerials unless you're in very specific situations (if you dtilt they shield and they roll away, you can cancel the dtilt with a sh into a max distance single fair to try to hit them/their shield and see how they react. marth is less about attacking and more about seeing how they react to things and waiting for mistakes, taking up little bits of space and defending your own space properly